Computer help forum: Computer screen shuts off randomly in-game when plugged in?

Hi everybody,

I've been running into a big problem recently when im playing certain games (LoL, Arma2) My screen will flat out shut off (actually i think its on and just blank) and my audio will sometimes scratch onto a loop. clicking, alt tabbing, ctrl alt dlting all do nothing, and i have to hard shut off my laptop and restart it to do anything.

At first I thought it was a heat problem. so i cleaned out the entire laptop of all its dust, applied new thermal paste, and even got an extra fan running outside of the system. After looking at a heat monitoring program, there didn't seem to be any problems, so I don't think this is the case.

Then I noticed something after fuming about not being able to play my favorite games with my friends. When I was on battery power, the shut off never happened. When i plug it in, It only takes about 2 or 3 minutes for it to shut off. Could somebody give me any ideas of what's happening?

I have a gateway p7805u, about four years old (but actually still runs high end games pretty well). nvidia 9800M GTS with most updated drivers. 8gb ram, 265 gb harddrive with about 60 gigs free.

Thanks so much!