I tested the ram with mem test 5.1 and it found no errors . I tested the hdd with hdd regenerator 1.51 , found no bad sectors . Recetly bought a new power supply 500W . Im using nvidia GTS 250 and i thought it was the graphics card but i borrowed my friends GTX 650 and it same problem the computer restarts randomly from 15 mins of use to few hours . Ive got like 2 chassi fans and 1 cpu fan so idont think its overheat problem , maybe its the motherboard or the cpu it self . The pc is rather old like 7-8 years and im thinking might it be the motherboard ? or the cpu?
All fans(including the CPU's) and heat sinks are dust free? The issue started before you purchased the PSU?