Very Wierd PRoblem

when i am playing any kind of game like counterstrike source im in the middle of playing and the computer stops running or i guess shutdowns. the werid thing is tho, the keybaord is still running. also when i move the mouse it starts up again and boots to windows desktop. what kind of problem is this?? plz relpy asap

also my specs are

a8n-E motherbaord
athlon 64 3200+ 2.0 from an emachine
a ati radeon x850 xt saphfire version video card
2 gigs of corsair xms version ram
also a 550 antec power suplly. farely knew.

also i just put a new motherbaord into my computer.
my old motherbaord was a emachine motherbaord that came with my old emachine.

this is the only problem i have with it now.

 

plz reply

 

so...while you are playing, the computer shuts completely off or enters standyby mode? you can move your mouse to bring it out of standby? does it go straight to a black screen or does it tell you the computer is shutting down?