VPN subnets keep dropping

[slave2thegrind]

Ok here's the problem: I have a branch office connection set up with another facility. I am running a Nortel Contivity VPN Device while the other end is running a Cisco device. They are allowed to reach multiple subnets on my side. But every once in a while for no rhyme or reason one ONLY 1 subnet decides to drop off. Meaning one of us has to bounce the connection. At the same time the other subnets that they are set up to access are perfectly fine. So... Last week I "nailed up" the connection. It seemed to work for this one subnet. But now a totally different one is dropping. All of these subnets are set up in the same branch office and the entire group is nailed up. Any ideas are welcome cause at this point I'm running out of'um.

Thanks in advance.