This sounds to me like an ARP issue, have a look at the ARP tables before and after the problem occurs.
One point I come across before is the PIX runs proxy ARP by default, turning this off can resolve some odd connection problems. Its a sysopt command to disable it, sorry cannot recall the exact syntax, do `show sysopt' to view.
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...