whats in this computer? is the power supply on top or below?
if its on top it could be absorbing all the heat from the cpu and video within the case.
iv seen this overheat low quality 120mm fan having psu in a top mount case.
due to rather hot running athlon 5000+cpu and a large hot hd3870 videocard.
gaming heat would cause psu to shut off.
to combat it iv installed an additional 120mm side panle fan an 80mm exhaust fan
aquired a 3rd party gpu heatsink "Tt duorb" and to make sure it never ever needs more internal air movement i threw in a dual fan card on the bottom of the case lifting air up and in from under the case...