Dennis found an issue where even after applying ISA 2004 SP1 on top of Windows Server 2003 SP1, the RRAS service repeatedly restarted. He contacted PSS and found that there is another small issue causing this fault that can cause packets to drop and is caused by an MTU issue.
Symptoms are like:
Inability to connect to terminal servers or to file share access.
Failure of domain controller replication across WAN links.
Inability of Microsoft Exchange servers to connect to domain controllers
Heres the resolution and it requires a hotfix: http://support.microsoft.com/?id=898060