Both my brothers have Dell XPS 420's and both are having the same issue.
While playing games or doing any kind of heavy load the computer will just shut down. There are no over heat logs in the BIOS and when I try to replicate it I can't. The CPU never gets to hot and it doesn't shut down.
On one machine I have replaced the mobo, memory, video card (HD 4650), power supply and hard drive to see if it was one of those causing the problem but it is still happening.
After replacing the video card anytime he plays StarCraft II, after a couple of minutes the screen goes black and the computer is frozen. The computer with the stock video card doesn't have that issue.
They will both shut off after playing games, like World of Warcraft, for about 20 minutes.
I can't figure out what is going on for the life of me.
Also, what is up with the XPS having a locked down BIOS? I thought these machines were made for gamers that liked to tweak and overclock?