VPN Disconnecting every 40mins or less

Hi

I have a windows 2008 R2 enterpreise server, and Routing and Remote access is installed.

VPN Ports are open in Firewall

users can connect to server using VPN.

Problem is that once they connected, they get disconnected every 30-40 mins time or if not disconnected, then they can't access the network on the vpn site.

can't ping

can't RDP

I have build second server and it is doing the same thing..

any suggestion?