I have a VPN to VPN connection from a remote property to my corporate office. The remote site has a PIX515E. Every 20 minutes clients at the remote property lose connectivity to the corporate office. If I attempt to ping the inside interface of the remote PIX, the clients can again talk. This is not the only PIX I have deployed at remote properties so when I compared configurations I was surprised to have not noticed any differences. My solution right now is to keep a continuous ping from the corporate location to the remote one. That seems to be working, but it's not a fix. I need to find the fix.
You could try to do a "crypto isakmp keepalive." It will send keepalive packets across the tunnel. It's pretty much the same thing that you're doing with ping. :-)
