Nasty one this - Hyper-V on Windows 2008 R2 just wouldnt give a stable network connection for remote desktop to Guests, and the host sometimes.
I spent hours searching for a solution on the net, fiddling with Netsh, MTU sizes, switches and NIC drivers, until I spotted this blog entry. Turn off the Large Send Offload setting on the NIC. POW Fixed. Thanks to Daniel Petri. No reboot required.