We had to downgrade a user who had continuous problems with that same thing. The following thread helped me a lot.
http://www.blackberryforums.com/show...&highlight=3.8
Hope this helps. BTW, we have an open issue with RIM regarding the Cingular 7290's and the 4.X OS causing the device to reboot itself or give a Device Error: 411 (note: have scanned the users' email for corrupt items [none found], replaced the device, did not put backed up items back on the device, deleted user from BES/readded, etc.). No ETA on the resolution - supposed to be a known issue that they are working on.