What version of client and router are you running? How random is this? Is the tunnel up in use and in the middle of say sending traffic it drops? Or are they sitting idle? Are you running a routing protocol on your router? That message indicates that the tunnel on one side has basically dropped but the other side thinks its still up so it continues to send traffic.
Try removing the reverse-route from the router, if your not running a routing protocol, it wont be needed and I remember there being a problem with that if you have a accelerator card. You might wanna upgrade the code as well to 12.2.11T2.
Im not sure of the bug id that was assigned or if it was filed with this in 12.2.11T or if 12.2.11T2 will fix it. Open a case with TAC to pursue this on the bug and which code, if any, will fix it. You could always try 12.2.11T2 and see if it fixes your problem or check the release notes for that release to see
Table of ContentsIntroductionVersion HistoryPossible Future
UpdatesDocuments PurposeNAT Operation in ASA 8.3+ SectionsRule Types
Network Object NATTwice NAT / Manual NATRule Types used per SectionNAT
Types used with Twice NAT / Manual NAT and Network Obje...
Table of Contents Introduction:This document describes details on how
NAT-T works. Background: ESP encrypts all critical information,
encapsulating the entire inner TCP/UDP datagram within an ESP header.
ESP is an IP protocol in the same sense that TCP an...