Why does the network fall off on Hyper-V?

Question

Hyper-V

There is a physical machine with Windows Server 2012 R2 running Hyper-V. Hyper-V is running a virtual machine running Windows Server 2012 R2. The problem is that the network falls off on the virtual machine, disappears and appears at intervals of 5-15 seconds. If at this moment the grid of the real machine is loaded, everything works. That is, while I'm pinging on a real machine, whatever, the network on the virtual machine works as soon as I stop falling off.
The real machine has one Realtek PCIe GBE NIC. On the guest, one external virtual network.
What could be the problem?
I made another virtual machine with Windows 7, there are no such problems.