"The server is currently disabled" 500 Error on IIS 5

We are running running a site load balanced over 2x Windows 2000 Advanced
servers running Microsoft II5 5 and Commerce Server 2002 (Standard Edition),
every so often (upto 4 times per day per server) the site crashes
unannounced with the error:
"The server is currently disabled." - This appears as a 500 error

The quick fix for us is to unload the site application in either IIS Manager
or via component services or failing that doing an IIS reset.

Ideally i'd like to find the cause of the issue - which I suspect to be IIS,
but I am stumped where to start....

I have downloaded and run IISState on one of the boxes, but can't really
understand the output (it may already be showing a problem - but I can't
identify it!!)

Anyone have any suggestions?

Thanks,
Adrian