Desktop randomly shuts down without warning

[noneplusone]

Specs:
Windows 7 64-bit
Gigabyte GA-P67A-UD3-B3
Intel Core i5 2500K
Kingston HyperX 4GB (2x2GB) DDR3-1600 Kit CL9
Samsung Spinpoint F3 1TB 32MB 7200rpm SATAII
EVGA nVidia GeForce GTX460 1GB GDDR5 PCI-E
Seasonic S12II Bronze 520W 80Plus Power Supply
Dell U2312 monitor

I've been having an issue with my computer where it will shut down without warning and then reboot. There is no BSOD when this happens. It happens very sporadically as well. Typically it shuts down when I play a game (Dota 2 in general) but sometimes it'll do its first crash when it's idling. It generally tends to shut down the first time when I'm gaming and then it'll happen a few more times. But for the few days preceding this I've been playing Dota 2 and it has not shut down. I have checked the capacitors on the mobo and they all seem fine. I have memtested the RAM and no errors were found. I have also changed power outlets as I thought that might've been the culprit. I've formatted and reinstalled Windows 7. I've monitored temps and they are all perfectly fine both idle and under load. This computer is about 2 years old and I've never had this problem occur before. It's been happening for at least a couple of weeks.

I did discover that there seems to be an issue with the mobo coming into contact with the case. Initially, when I put pressue on top and back of the case it'd cause the comp to shut down. I removed the side panels and found that pressuring the top did nothing. So I poked ever so slightly at the piece of metal the mobo is mounted to and found that pressuring a certain area a little caused the comp to shut down. My guess is that the side panel which covers the bottom of the mobo is too tight and when the top is pressured whilst it is attached it must push the mobo into contact with the case. I'm certain I haven't been inadvertently pressuring the side when these shutdowns take place so I assume that this isn't the cause. And they seem to begin when I play Dota 2 (there doesn't seem to be any consistency in terms of how long I'll be playing for before it shuts down. I can play entire games and it's fine but then sometimes it'll consistently crash during 1 game).

Also, just today when it has shutdown it has not rebooted instantly as it was before. Instead 30 seconds or more go by and then it boots.

Does anyone have any idea what the trouble could be? It seems like it has to be a mobo or PSU issue. The fact that it crashes more when under load leads me to believe it is PSU but I am just guessing here.

Any tips or suggestions are greatly appreciated!

[Geek-9pm]

The fact that it crashes more when under load leads me to believe it is PSU but I am just guessing here.

Yes, you are guessing.

Replacing every component is a very costly way to troubleshoot.
Try this, not because it is likely, but because it cost only your time. :
Disable the floppy dive floppy drive. No power, not data.
Do the same for the DVD  drive.
Use just one stick of RAM
Remove nay USB devices not needed for basic use.
Disable any and all wireless devices. Radio off. No wireless mouse or keyboard.

As stated, it is not lively, but it cost nothing to try.

More likely is a component failure in the motherboard. But a new motherboard is going to be a pricey test if it turns out to be a bad DVD drive. Or RAM. So fist eliminate things that would let the system nun anyway. Try both sticks of RAM, one at a time.

[patio]

Were all MBoard standoffs used ? ?
You may want to get some extras....
How are you monitoring your Temps ? ?

[noneplusone]

Thanks for the responses guys.

Yep I have tried disabling my CD Drive. I have no floppy drive. I'm using only the necessary USB devices. I haven't tried only one stick of RAM yet. I'm pretty sure all the standoffs are in place. Don't think there are room for any others. I've been using CPUID HWMonitor to monitor temps.

[patio]

Pretty sure means you should check they are all in place...
Also  in Control Panel/System/Advanced you should turn off "auto-restart on errors"...and save the changes.
This means you will get the appropiate BSOD's which will help narrow down the issue.