I hate this kind of problem. I had a similar problem in a gaming pachine for the kids... It only did it when using a graphic intensive application (like a game) I ruled everything out, including swapping PS, except for the possibility of a flakey MB, CPU (amd) or video card (ATI)...
There is one thing I do different than anyone else I know. I don't put complete trust in system reported temperatures. Before spending too much time on any problem, I measure the temperature of every component I can see with an infra-red thermometer. Many intermitten problems are heat related.
My problem did not require massive time committment becuase it also provided a solution to another problem. It gave me a computer that no one ever wanted to install games on. It works perfectly good for everythint else. I simple added a second monitor to the video card (for my dual-monitor needs) and made it my PC and they use mine for their games.
If I were going to spend more time on it, I would suspect: (in order)
1. AMD CPU (I had no way to test it)
2. Video Card (I had no way to test it either - no direct swap spare)
3. MB (again, I had no exact replacement.
All of the above passed all diagnostic tools I own... but that's not 100% either.
Everything else I absolutely eliminated, including memory and HD.
I just fixed a Compaq laptop that was doing that exact same thing... after exhausting all reasonable diagnostics, I replaced the HD (it was a Samsung also) it worked. The owner was happy with the increased storage also.
If any of this helps, you are welcome to it. I don't normally put in my two cents worth because usually people have already done anything I might have to offer... which is pretty much the case here too.
You can have any of my typo's also. I didn't bother proof-reading.
