Originally Posted by Milez5858
A weird and complex problem indeed. However, since there doesn't seem to be a general outcry that IPNAT.SYS in win2k8 chews data, the most likely explanation is some as-yet-unknown environmental factor - a driver which is interfering, most likely.
I'd suggest testing what happens in [safe mode + net]. Afterwards, try disabling "chimney" (TCP offload), RSS, and NetDMA.
If all that still produces consistently wonky results, I'd beg/borrow/steal a different NIC as a test. While I realise that the results of your previous testing do not suggest any obvious hardware involvement, the interaction between all these kernel-mode components is very complex.