NVGRE Gateway stops forwarding packets

I've deployed NVGRE Gateway and added a virtual network using NAT and it works fine..

...about 15 minutes then everything stops. I have ping running to external IP address and it works fine when adding a NAT connection to the virtual network, but suddenly stops after approx 15minutes. No events can be found from the guest VM, from the GW VM, from the gateway host or from the SCVMM. 

Everything come back to normal when removing the NAT connection and re-adding it.

I have installed the hotfix 2918813 (http://support.microsoft.com/kb/2918813) on the GatewayVM but it doesn't change anything.

Any ideas how to troubleshoot?