9500gt shutting down computer help

ok specs first 3.33ghz celeron with 2gig ram and 300watt psu 9500gt pcie
also it is not overheating running around 50 celcius gpu and the cpu is little cooler than that 44celcius or so.also have a 8600gt that worked fine with this rig (in my cousins comp right now dont have on hand)and i am testing this 9500gt for my cousin need to get it working or send it back within another week

now problem works fine for most small games and watching on tv etc but when i try something bigger like halflife 2 and nfs carbon and so on (which i know it can handle)it will start the game and i get in the game,game working fine within a few minutes sometimes less the screen goes black and the sound goes choppy as if it froze and will not go back to windows after a little while the comp reboots or i have to manually shut it off.ALSO FOR THE LOVE OF GOD THE THING WILL NOT LET ME ADD LATEST DRIVERS AND WHEN I MANUALLY MAKE IT TAKE THEM IT THEN WILL NOT LET ME USE THE NVIDIA CONTROL PANAL?HELP AND THANK YOU