MS Exchange 2003 performance issues

We are running an Exchange server 2003,Enterprise Edition, that has to be rebooted every 4 weeks or so for performance reasons. It will process emails very quickly and slowly degrade over time until we reboot it again.
The server is 2K3 standard, quad processor and 4 GB of RAM.
We have run the suggested MS Exchange tools on it to make sure the configuration was correct. It isn’t doing any other jobs (i.e. DHCP/DNS, Sharepoint, DFS, etc.)

Any thoughts?