PC RESTART RANDOMLy

I M GTX

Honorable
Jan 2, 2014
26
0
10,530
hi guys , i am facing a weird problem with my pc . its restarting by itself randomly some time in 5 min
or sometime in 2 days or 1 week or some time its totally blackout not even boot up to win 7 logo . i tried everything to fix it like change PSU , change harddisk, change graphics card ,change dvd rom
nothing happened the result is same----- it is restarting ---but i did not change ram but i tested with all soft like MEM test and windows memory test . its really weird sometime no problem at all but some time worse . when i play games then it doesnot restart ....it mostly restart when pc is in idle state . pls help me out
my ps spec is

processor- core 2 duo 3.00ghz E8400
Mobo- intel dg 35 ddr2
ram- 4 gb ddr2 800 bus
harddisk - samsung 500 gb
gpu - zotac GTx 760 4 gb ( yea i know its bottlenecked and i m planning to upgrade my pc within 3 month )
PSU - tharmaltake toughpower 650 watt gold

its definitely not power issue coz i tried different psu and the result is same .
PLLS GET ME OUT OF THIS MAZE