Hi, could someone recommend a good program that will put the computer under stress for a given time?

I have done some adjustments to RAM, Video, CPU & Bus speeds both in Bios & XP, all goes ok except after playing SHIII for around 1 hour the computer then locks with video displaying hazy lines & a buzz in the speakers, it's bad enough to change settings let alone play SHIII for an hour or so to test!

Any advise welcome.

Thanks,
Reece.