Powerdown Problem

So I'm an online gamer. For the last 6 or so months I've been playing World of Warcraft (WoW) with no problems on my Inspiron 8500 (512mb RAM, 2.20GHz Mobile Intel Pentium, Mobility Radeon 9000). Earlier this week, I started getting a weird problem. After a solid hour or so of playing the game, my laptop turned itself off. It was the type of powerdown that was immediate, like holding down the button to turn it off. Its usually preceded by a few seconds of thick lag in the game. Since this started, it happens every time I load WOW. Sometimes I can get a few minutes of play in before it hits, sometimes its immediate after the game loads, but it always hits -- the laptop always turns off. Only happens with WOW. I've tested other programs too. Going to load some other graphically intensive games soon and try with them. I have an active, up to date virus scanner which showed no problems on a scan yesterday, and I also defragmented the drive in case that was a problem.

I've made no changes to the system, installed anything, dropped it, or anything else I could think of that would cause bigtime problems. I don't believe it to be overheating as I have a nice laptop cool pad that has extra fans on it and I don't recieve the overheating message when the laptop boots up (and its barely warm to the touch). It starts up like any other time, as though nothing happened.

I also posted up something on the game's forums, but since I can't tell what type of problem it is, I'm also posting the deatils here in hopes of a speedy response. Also, I believe my 3 year warranty has expired by now.

Thanks for any help anyone can provide.