I've heard of a similiar issue with IOS software ver 12.
Depending on the version you're currently running, you may just want to check for things like memory exhaustion (sh block); Stateful failover - lu allocation failure; Translation tables filling up or the processes run time being too long.
Excluding of course the 557poll.
If you only deploy GD images, you're left with only one option, that being 6.1.4
You may than want to double check the caveats against your own environment.
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...
[toc:faq]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 and UDP are I...