I came to that conclusion because when im playing Grid or cod5 with fan on automatic after 15 -20 mins of playing my fps drops and it lags/stutters.
with the fan on at 50% it plays fine for hours (several)
idle around 32, load around 50-55
Maybe my power supply isnt giving enough amps

its 550watts no idea about the amps thou.
EDIT: PSU is 500W not 550, It had 18a on 12v & 12v2 lines, i suspect this isnt enough