You have probably solved the problem already, so I may be too late with this suggestion:
One of my customers had a similar issue which we were able to resolve by hard setting the ethernet interfaces to 100Mbps and half duplex. It seems the ASA does not always auto-negotiate correctly causing the links to become flakey.
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...