Exchange 2000 performance problem

Hardware: Proliant 380, dual 750 Mhz processors, 2GB Ram
Server Software: windows 2000 SP4, Exchange 2000 SP3
Client Software: Windows XP, Office XP, latest service packs.
We periodically experience delays of from several second to several minutes when someone launches their outlook XP client. A pop up window occurs indicating that data is being downloaded from the server. I have worked with Microsoft support for 6 months on this single case. They have finally escalated it out of the support engineers area, but the problem still remains unsolved. During the delay, CPU and disk utilization on the exchange server are low. Microsoft and I have taking network traces, log traces, etc. to no avail. Any thoughts on how to troubleshoot this problem or of a resolution? Thanks