Your symptoms certainly sound like the IPS is dropping packets in your VPN connection.
Check your sensor event log with:
show event past 01:00
to see all events in the past hour, some alerts (sh event alert past 01:00) are supressed, depending on the signature settings. If you can determine which signature is responsible, you can disable the signature, or remove the drop action. Keep in mind that any signature with a high risk rating (80 or better?) gets dropped automaticly, reguardless of the action setting of that signature.
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...