Parallels Unexpectedly Quits / Disappears

I am not sure if this is related to problems others are having with Parallels shutting down and OS X reporting "Application Parallels has unexpectedly quit ...". My setup is Parallels 5160 on a 2 GHz MB with 2GB of RAM and a 30 GB XP SP2 installation. Networking is bridged. After several hours of use (usually overnight), Parallels disappears - no quit dialog, no notification from th OS. Sometimes, the VM scrteensaver is active and when I mouse over the area, Parallels disappears. My VM can be restarted and it restarts from a suspended state.

I have uninstalled and reinstalled both 5160 and Parallels Toos for 5160. The problem persists. I have looked through crash logs (but I could not find one for Parallels) and I can not find a cause. The problem occurs whether or not the MB is using wired or wireless network access.

Any suggestions?