TFTP block ACK issue during PXE boot WDS

WDS pushing PE boot wim for workstation imaging. Everything else coming from the server is transferring fine (normal speeds), but the wim is incredibly slow (~35min 2GB file). Wireshark shows this interesting bit of interaction where the server re-sends first packet in the block at the end of the block causing it to fumble. So instead of

server sends packets 1,2,3,4,5 client sends ACK 5 

it goes

server sends packets 1,2,3,4,5,1 client sends ACK 4 

Here is a screenshot of the Wireshark captures. This doesn't happen on every block, but it happens enough to cause the slowness. Normally the delta between each packet in the block is ~20 microseconds, but when the block gets screwed up, the last packet the server sends in the block takes ~2 seconds. Now multiply that by 1000x it happens during transfer session and it quickly adds up.

I've tried:

• reinstalling WDS
• changing TFTP block size
• removed the Windows update from March 19 just in case
• reducing the wim size

I couldn't find much on this particular issue, so any help is much appreciated.