Another firewall sits inside the PIX525, then out to the internet. A L2L VPN through the PIX525 hangs every few days and is recovered by rebooting the PIX525. The end peers report "IKE Responder: Remote party timeout - Retransmitting IKE request" and "IKE negotiation aborted due to timeout", the PIX525 reports "%PIX-6-110003: Routing failed to locate next hop for UDP from inside:a.b.c.9/500 to inside:[remote_peer]/500".
Note the "inside:[remote_peer" - this peer is actually outside and PIX525 even has static host route for it:
route outside [remote_peer] 255.255.255.255 a.b.c.1 1
When this happens PIX525 can actualy ping remote_peer.
Sometimes this happens several times a day, sometimes it goes 5 days without issue.