NEW BUILD ASUS P5Q-E

Hi guys i have built a new rig.The issues im having here is that whenever i fire up a game after userally about 30-45min into a game the games lock up.
freeze. heres what i have done.

ive installed the latest drivers.and old drivers for mother board and even video card. no joy
ive tryed two video cards ati 4870. i used fine for year with old system .and brand new ati 4870 1gb version not the 512.witch this system failed on possibly cos the psu. but didnt fail on the 512mb

ive tested 2 hdd's

ive had win xp pro. 32bit
ive also had win7 64bit.

ive ran gpuz witch showed my vga at 50-60 temp on load
my memory fine

ive ran benchmark tests witch seem to pass fine too thats inc 3dmarks 06

but if i run games locks up. heres whats under the hood

ASUS P5Q-E
ATI RADEON 4870. 512MB
A CORE 2DUO E670 2.66 GHZ NOT CLOCKED
2X HDD'S SATA 160GB AND 1TB
2X1GB HYPERX DDR2 1200
DVD OPTIC WRITER
AND A HIPPER 730WATT PSU R TYPE

NO LED'S LIGHT UP ON THE VIDEO CARD WHEN IT CRASHED BUT FLASHES ON AND OFF WHEN REBOOT MY COMPUTER OR TURN ON SO GUESS THATS NORMAL. HOWEVER I DID BY THE PSU AND BOARD 2ND HAND SOMERT I NEVER USERALLY DO. THIS WAS MEANT TO BE AS GIFT FOR MY FATHER NOW HE OWNS MINE.
NOT SURE WEATHER MY BIOS IS SETUP WRONG BUT BASICLY ITS ALL ON DEFAULT.
IM HOPING ITS THE PSU AS I SOONER REPLACE THAT THEN THE BOARD. ANY HELP GREATLY APRECHIATED