Computer turning off at random during video game play

[Sketch]

I purchased a computer back in 8/08 from CyberPower PC, before I knew how to built them myself. It's been running practically flawless since purchase up until recently. The problem occurring could definitely be due to age but thought I should throw it up on here for some assistance.

Over the past couple days my computer has been randomly turning itself off during game play(like the system has been unplugged). About three times in Aion and once so far in Bad Company 2. I thought it was due to overheating or dust or whatnot - so I've cleaned up all the dust internally and set it up on a table leaflet so that it isn't sitting on carpet anymore. After I gave it a thorough cleaning it worked perfect for about a day and a half and just recently went out on me again. After it goes out it won't allow me to attempt to boot it back up for at least 10+ minutes. Could this be due to overheating still(I have never had an overheating issue) or due to my psu going bad or getting old? Also was concerned about my cpu chip and if it was overheating.

I also made sure driver's were updated all around(had been having some graphical errors) and everything seems to work perfect. At one point I had SLI'd cards in my computer but one of them seems to have burnt out. My computer will no longer recognize it and if I make it the top card in my mobo it will blue screen with graphical errors.. so I removed it. Been working fine since it's been removed. Compy stats listed below:

Running Windows Vista 64-bit
Coolermaster Cosmos Sport Case
EVGA NForce 780I 775 mobo
Intel Quad Core Q9550
Thermaltake Big Typhoon VX/VR
2GB PC6400 DDR2 800 x2
EVGA GeForce 9800 GT 1GB 16x PCI-E (used to be two, now only have one)
750GB Sata-II 7200 RPM 16mb
black Sony 20x DVD/RW
Apevia Warlock Series 900w PSU

Any suggestions would be extremely helpful! It's driving me insane.

 

[hellokitty[hk]]

Eh...why don't you check your temperatures?

 

[Sketch]

Sorry about the delay in posting again. Things were working alright for a while

I did check my temperatures. They were a bit high so I increased my fan output. Now they sit just fine. Everything under 50c, everything increases just a little during game play. vid card sits around 55 during game play.

Still shut off on me in the middle of playing Civ 5 last night. Again wouldn't reboot for a while - got irritated went to sleep.

EDIT: Went back into Civ 5 to see how my temp's are responding. My vid card was fine and getting around 55c in game - as the game went on went up to 75c, it didn't crash yet though. I closed the game to let it cool off... but I thought it was quirky that PCWiz said it dropped almost 20 degrees in about 2-3 minutes. Is that correct?