I had some problems with cached mode in outlook 2003 also on the server
sbs2000 i enabled the deletion settings to 45 days.
It so happened that the user had some messages that were 40/120mb in size
emails.
I used the filter on the deleted items folder and set it to anything less
than 5mb in size.
So that's how i fixed my cached mode problems.
hope this helps someone.
webboy