Update: Newer firmware has been released since this was originally posted.
Ran into an interesting issue last week with a HP DL 585 Hyper-V host which had intermittent network connectivity over one of its defined virtual networks.
In the system event log the following warning was logged:
To resolve the issue, HP has released an updated NIC driver which is dated 11-11-2011.
The below is pasted from above to facilitate easier searching:
DEVICE: HP NC375T PCI Express Quad Port Gigabit Server Adapter
PROBLEM: Tx path is hung. The device is being reset.
ACTION: Adapter recovers from this error automatically.
Version: 188.8.131.522 (11 Nov 2011)
The driver update is downloadable from here.