Sounds like a heat problem to me.
To test this, take the side cover off the system and see if it either eliminates the problem, or at least seems to delay its onset significantly.
This would also be a great time to get out the compressed air and blow any dust bunnies out of the internal fans.
I have a dell 8200 with windows sp3 and I experience random freezes when surfing the internet. I can surf maybe a few minutes before it freezes (mouse no longer moves and computer doesn't respond to any commands. I have to hold down the power button to restart). This happens fastest during streaming downloads (about 5- 10 minutes before freezes). With my web browser up (switched to Firefox but it didn't help) I can get about 20 min of surfing. When I'm downloading with no browser up I can get a few hours but it freezes eventually. When I'm connected but not doing anything it seems to stay fine indefinitely. I'm completely out of ideas. PLEASE HELP!!!!!