NT Network and VPN connectivity issue

I have 2 NT networks connected via Cisco software VPN. Two Cisco 1720 routers. I lose connectivity on occasion. I cannot ping or telnet into the opposing router to check configuration when this occurs. I can ping and connect to a file server behind the router. Rebooting the router does not help. It`s as if something is interfering with the operation of the router on occasion. This problem clears itself and connectivity is restored. I have had the router disconnected from the rest of the network and tested ….same result. Any ideas?