Gears of War PC shuts down my machine

Been playing some good old gears of war (PC) with my bro. I´ve been hosting these LAN games but now all of the sudden my pc shuts down on certain areas and now I can´t pass one. I then try UT3 with him and works fine. and so I try running Crysis (on high settings)thinking perhaps my gpu or cpu is overheating or something and crysis works fine too. I dunno now if I can run crysis and other games well why does Gears of war making my machine shut down? has anyone heard of this? btw me and my bro have a 4870x2 I have a quadcore 9300, x48, 1000watt psu, 4gbram he has a corei7, x58, 3gb ram, a 750watt psu, and he has no problems what so ever. I have tried on both vista and xp and it´s the same drama. I´d be pleased to hear any of your suggestions!

Hmmmm....I really don't know. But I was experiencing the same problems on GTAIV, until eventually it became unplayable because i couldn't get to an area which i needed to get to, I have a 1gb 4850, and if I was to check the CC it was 55C at max.

yah i was having problems like that with fallout3 but with patch it was all good

like shut-down as in lose power? or in like u get Blue screen? if u get blue screen, right down error and google it, if not blue screen, reinstall gears maybe?