Q: My Hyper-V virtual machines lose network connectivity and I have to restart the VMs. How can I fix this?

A: I hit this exact issue, and a fix is available for Windows Server 2008 R2 and Windows Server 2008 R2 SP1 at Microsoft support  and at the Microsoft download site respectively.

It's also available on Windows Update as an Optional Update.

Once you apply it, you shouldn’t have an issue with VMs losing network connectivity.