M17xR2 random shut downs

Alright, recently been having problems with my computer but have gotten most of them solved. Infact all but one:
About once a day, the system will just shut down. Doesn't BSOD, doesn't lag before doing so, it literally just shuts down as if I had held the power button for 5 seconds.
I hit the power button and it powers right back up.
Within the past month i have reformatted and reinstalled windows due to other issues I was having.
I installed a 7970m a little while ago and the only thing I can think of is maybe the 240w power supply I have isn't enough to run my 7970m and 920xm sometimes? Anyone else with a 7970m using the stock 240w power supply?
I'm told it should run it just fine with a single 7970m, so what are other possibilities to cause this?
I haven't noticed it do it while idling or web browsing to be honest, seems to happen mostly during game sessions.
My temps are fine though, CPU and GPU doesn't exceed 75c under max load

 

Note it seems to mainly do it on first boot, if I shut it down for the night, turn it back on the next morning, when I launch any game it'll crash after about 10 minutes, than when I reboot it from there, its fine for the rest of the day usually.

 

Since you reinstalled you OS and it still shutdown like cold stop/start and you installed the 7970, my guess is that it's a power issue.
I suggest that you lend/buy a voltcraft meter (or something simular) and see how much W gets pulled from the socket.

If this is not the issue, i think its HW failure.
Try placing you old GPU back and see if the problem still occurs.