Hi
I think trying to find the common link between a number of infrequent errors is always difficult. However, memory leaks tend to show themselves with prolonged use (although not always). Also, if this was the cause then we would see it on all platforms, and Windows XP would not be excluded.
Although it does not use RichEdit, our server software runs for weeks without fail (it would be months if it were not for the Windows monthly updates requiring night-time reboots). We do not see any memory leaks, despite the automated attacks and the strange things some proxy servers do.
So to return to your point, I presume you are delivering the tests using the Question Tools Exam software. The most likely cause is something different about the systems on which they are run. It could be something missing, or something extra that has been installed.
A screenshot might be useful.
Hope this helps (as far as it can).
Regards
Paul Booth
Question Tools Support