This has shown up on other WM2003 devices as well. For the record, my wife has an X5 that started with PPC2k2 and I upgraded it to WM2k3. I have a newer X5 that came with WM2k3. Neither of us has run into this problem. However, I bought two X5's for a married couple we're friends with and BOTH of them had the same problem. It was solved by going here:
http://www.burroak.on.ca/wm2k3tweaks.html
and downloading the utility, running it and configuring it (it basically does some registry setting changes and allows cleaning of some of the database tables in PocketOutlook.
Microsoft SORT of acknowledges the problem, but isn't putting resources into it because of the apparent random nature of the problem. Personally, I wish they'd release a complete detailed breakdown of every possible registry key and setting that WM2003 can have. I'm not timid about registry surfing, and there's a lot behind the scenes that can be adjusted.