PC restart while playing games

hi

i bought my new pc about a month ago ..
i got
i7 920
MB Asus P6T
Corsair Ram 3GB @1333
PSU Silver stone 650w
GPU EVGA 275GTX Superclock

i am having a problem that my pc restart while playing games ...i check everything but dont understand what is the problem ..  
nothing is overheating everything is working on default clocks ..nothing is overclocked .....
first my brother was playing Mass Effect 1 when thins start happening ....i was thinking something was overheating i increased GPU Fan speed then it was fine ..
but next day i was playing Age of Empires 3 again my pc restart ...then i do some cable management then everything was fine ...
now it is started again after few days ...
i was playing crysis first it was fine for about 3 hours then a restart ..i started crysis again but this time pc restart after 15-20 mints ....
pc is restarting after 15-20 mints while playing games....
GPU max 66c @80% fan speed
CPU max 78c and AVG is 70c for all cores





here is the image ..
i test it for about 30mints and nothing happen ...and voltages were normals ....
i need a software that makes logs for CPU,GPU and all voltages ...this will help ...me to see what is happening while playing games ..
HWmoniter dont have the option for logs and real temp only make log of gpu and cpu...