What graphics card are you using ?
Did you apply the thermal paste/compound to the cpu ?
Is your psu big enough ?
Have you tried running the games with just one stick of ram and alternate the ram sticks to eliminate this possible cause.This assuming than more than one stick of ram.
What about cooling ?
Any case fans ?
Is the psu fan working ?
is the heatsink working ok ?
Sorry about all the questions,but the answers may well help us help you.