I still dont know whats actually causing the problem, so its hard to say.
Are you PC-literate enough to get the task-manager's 'cpu-usage graph' to display?
If it is going over 75% when you run the level, it may be too much for your machine, or MOST
likely youve got a malware problem.

I also have no idea of the impact Vista may cause if you have it.

NOTE TO ALL: I have never been able to re-produce this problem except through faulty coding...