(1) I assume you are using dynamic NAT and PAT in order to ensure that when your NAT pool is exhausted then all additional "users" will share the same external IP using PAT?
(2) There is a limit of 256,000 NAT/xlate entries on the FWSM. One thing you can try is the "xlate bypass" command which prevents non-NAT sessions from being included in the xlate tables which may also help.
See the documentation here for how to do this with v3.2:
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...