That's odd that there are no entries for the WWW service entering a stopped state or reason for a crash.
One thing I would suggest is install and run DebugView on the exchange server that your're having this issue with: http://technet.microsoft.com/en-us/sysinternals/bb896647.aspx
With that you can run a dump trace on your server to see if there are any clues as to why the WWW Service keeps crashing.
If all else failes, You can resintall the WWW service to see that corrects any issues.
Rafael Marrero
Technical Support Representative
www.serverintellect.com Managed Servers, 24x7 U.S. Support, Web Hosting Solutions.