Have been having a bit of an interesting issue over the past few weeks whereby our Hyper-V Hosts (Dell T430 Tower Servers) would loose network connectivity at seemingly random intervals; the only resolution was to restart the server or to remove and replace the network cable.
After much investigation looking at the servers and associated network switch we discovered that only the Virtual Switches attached to the on board Broadcom NetXtreme adapters were having issues and that the Intel PCI card NICs were not.
That soon lead onto Microsoft KB 2986895 which relieved a known bug in the drivers for the Broadcom adapters that messed up the Virtual Machine Queues (VMQ) feature of Hyper-V causing a loss in network connectivity. The fix is either to update the driver to a version that does not have the issue or to disable VMQ.
More details can be found in this Microsoft KB… https://support.microsoft.com/en-us/kb/2986895