Multicast falls back to Unicast HRESULT = [0x800705B4]

Hi all.
It seems that Multicast is failing and falling back to Unicast, even when we have only one computer being imaged at a time.
It can do this at 1% or anywhere between to 30% etc. Sometimes - just every now and then - it completes successfully to 100%.

I can't figure out why this is happening.
We are only getting up to 5 or 6 Mbit/Second at best on 100Mb switches (client end).
There are no collisions etc on the switch. Ports and client set to auto neg.

Below is an extract of the setupact.log on the client.
Any help on how to resolve this issue and keep multicast up would be appreciated.
I can't find info on the 0x800705B4 results anywhere on the web related to WDS/Multicast/Unicast.

2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Multicast Client: Server=[192.168.250.169], Namespace=[WDS:Laptop/lapcap-(2).WIM/1], Content=[lapcap-(2).WIM]
2009-12-10 14:05:45, Info       [0x0b008c] WDS    CClientLibrary::InitializeRpcSession: Binding String=1a927394-352e-4553-ae3f-7cf4aafca620@ncacn_ip_tcp:192.168.250.169
2009-12-10 14:05:45, Info       [0x0b008c] WDS    CClientLibrary::InitializeRpcSession: Domain Joined=No
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Session Details: Multicast Address=[239.0.0.211:64738], Server Address=[192.168.250.169:64738], Content Size=[1610158], Total Blocks=[184], Block Size=[8785], Session Id=[0x799360f0], Metadata Length=[0], Security Mode=[0x10001], Sign Key=[No], Symmetric Key=[Yes], HMAC Algorithm=[32777], Hash Algorithm=[32780]
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Using Local Interface 192.168.73.44 to communicate with server.
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    UDP Endpoint[Data] - Local=192.168.73.44:64738, Multicast:239.0.0.211:64738, Unicast:192.168.250.169:64738
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    JOIN-ACK: ClientId=0x425aa38c, RTT=16, MinNACKBO=0ms, MaxNACKBO=0ms
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    QCR Backoffms
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    New Master Client as reported by server: 0x425aa38c
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:45, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:46, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:46, Info       [0x0b00b0] WDS    Using backoff time 75ms before processing POLL packet.
2009-12-10 14:05:46, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=0
2009-12-10 14:05:46, Info       [0x0b00b0] WDS    Flow Window initialized - Starting Seq#=1
2009-12-10 14:05:46, Info                  WDS    Multicast transfer: Transfer Session started.
2009-12-10 14:05:46, Info                  UI     CallbackServerWaitDone happened
2009-12-10 14:05:46, Info       [0x0a0134] UI     Accepting Next. Exiting Page Waiting for Server.
2009-12-10 14:05:46, Info       [0x0a0035] UI     Allowing Page '' to be shown
2009-12-10 14:05:46, Info                  UI     Entering Page Progress.
2009-12-10 14:05:46, Info       [0x0a0035] UI     Allowing Page 'Waiting for Server' to be shown
2009-12-10 14:05:46, Info       [0x0a0035] UI     Allowing Page 'Waiting for Server' to be shown
2009-12-10 14:05:46, Info                  IBS    PublishCopySetupFilesProgressEstimates: CopySetupFiles time estimate is [0] seconds.
2009-12-10 14:05:46, Info                  MIG    Callback_Estimate, It is not upgrade mode.
2009-12-10 14:05:46, Info                  WDS    PublishWdsTransferProgressEstimates: WDS transfer time estimate is [1548] seconds.
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\CopySetupFiles  Estimated secs: 0  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\CopyWdsFiles  Estimated secs: 1548  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\TransferFinalImage  Estimated secs: 960  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\SkuAssembly  Estimated secs: 600  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\LanguagePack  Estimated secs: 540  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\CBSServicing  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\InstallingDrivers  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\CMIOnline  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\SysprepActions  Estimated secs: 960  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\Specialize  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\CleanupTempDrive  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\DeleteBTFolder  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a0030] UI     Reading estimate from blackboard: ProgressInfo\SendDiagnostics  Estimated secs: 300  Completed steps: 0
2009-12-10 14:05:46, Info       [0x0a011c] UI     WizardDialogPost::SetActive
2009-12-10 14:05:46, Info       [0x0a0023] UI     progress task ProgressInfo\CopyWdsFiles : StartProgress
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    Delay before sending LEAVE packet: 17ms
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    Multicast Client: Server=[192.168.250.169], Namespace=[WDS:Laptop/lapcap-(2).WIM/1], Content=[res.rwm]
2009-12-10 14:05:52, Info       [0x0b008c] WDS    CClientLibrary::InitializeRpcSession: Binding String=1a927394-352e-4553-ae3f-7cf4aafca620@ncacn_ip_tcp:192.168.250.169
2009-12-10 14:05:52, Info       [0x0b008c] WDS    CClientLibrary::InitializeRpcSession: Domain Joined=No
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    Session Details: Multicast Address=[239.0.0.37:64862], Server Address=[192.168.250.169:64862], Content Size=[6928163671], Total Blocks=[788636], Block Size=[8785], Session Id=[0x799360f1], Metadata Length=[0], Security Mode=[0x10001], Sign Key=[No], Symmetric Key=[Yes], HMAC Algorithm=[32777], Hash Algorithm=[32780]
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    Using Local Interface 192.168.73.44 to communicate with server.
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    UDP Endpoint[Data] - Local=192.168.73.44:64862, Multicast:239.0.0.37:64862, Unicast:192.168.250.169:64862
2009-12-10 14:05:52, Info       [0x0b00b0] WDS    JOIN-ACK: ClientId=0x425aa38d, RTT=1, MinNACKBO=0ms, MaxNACKBO=0ms
2009-12-10 14:06:22, Info       [0x0b00b0] WDS    No activity for long time: 30046 ms.
2009-12-10 14:06:22, Info                  WDS    Multicast transfer: Transfer Session done. HRESULT = [0x800705B4]
2009-12-10 14:06:22, Info       [0x0a0024] UI     progress task ProgressInfo\CopyWdsFiles : EndProgress
2009-12-10 14:06:23, Info       [0x0b00b0] WDS    Delay before sending LEAVE packet: 62ms
2009-12-10 14:06:23, Warning               WDS    CallBack_WdsClient_MulticastCopyFiles: Error transferring files. Falling back to unicast. Error [0x800705B4]
2009-12-10 14:06:23, Warning    [0x0b0095] WDS    CallBack_WdsClient_MulticastCopyFiles: Multicast transfer failed. Error code [0x800705B4].