This is obviously a tricky one.
My only advice would be to try and eliminate as many possibilities as you can:-
Are you running a screen saver that may be causing problems? If so, disable it.
Does it only happen when certain applications are running? Try locking it when you have different applications running, to see if that makes any difference.
Does your PC go into any sort of standby mode after a certain period? See if there is a pattern regarding the amount of time you leave it, before unlocking.
Is anything appearing in the event log?
These are the obvious things to check, but to be honest, it could be any application causing the problem.
Good luck.
Rik