The original post described this as a GRE tunnel. And the suggestions made in the posts responding are pretty much based on the assumption that this is a simple GRE tunnel. But the added observation which mentions ISAKMP makes it fairly clear that this is not a simple GRE tunnel but is a tunnel with encryption. This makes it a significantly different situation. I have seen many situations with an encrypted tunnel that goes down and it turns out that what causes the tunnel to go down is related to the encryption.
To offer helpful advice we need more and better information, starting with the configuration related to this tunnel and to the encryption.
Without having that information I will offer my first guess at what is causing this behavior. My guess is that initially there is interesting traffic, the presence of interesting traffic causes negotiation of the IPsec SA (including its lifetime) and that brings up the tunnel. The tunnel stays up during the negotiated lifetime. When the lifetime expires there is not interesting traffic to negotiate a new lifetime and the tunnel goes down. One way to verify this would be to bring up the tunnel and then to use commands to show the IPsec SA. Repeat these commands from time to time and watch the lifetime. As the lifetime gets toward zero see if the tunnel comes down. You might also get verification of this using debug output for IPsec negotiation.
HTH
Rick
HTH
Rick
