Services unexpectedly terminate

I have Exchange 2000 running on a W2K server. At random times throughout the day and night, several of the Exchange services, the IIS Admin service, and the WWW service terminate unexpectedly. I set the services to restart automatically, but afterawhile teh POP3, IMAP4, and SMTP services refuse to start up and I am forced to reboot. Everything seems to work fine when the services are up, but I am looking for ideas to try and cure this problem. Only Exchange and IIS services are affected. A quick response would be appreciated.