I currently have approx. 20 remote branches on cisco 2811 routers connected as spokes to a Cisco ASA 5510 at the head-end. These are simply stright ipsec connections with a shared-secret. All the routers are running 12.4(15)T IOS. I have a probelm with on of my sites that the router will not pass any traffic. This happens on a random basis. It may go days but then traffic will stop flowing from the branch to the head-end. Once I do a clear crypto isakmp traffic starts to flow again. None of my other routers are doing this. I've set the isakmp keepalives. What is interesting is that the tunnel is still active but the traffic does not pass until I clear the tunnel.
I have this problem too.