I'm fairly sure this is a power supply problem, but it is very strange. After playing Alien Swarm for about 5 minutes, my computer shuts itself off as if the power went out. It only happens in Alien Swarm. According to speedfan, nothing's running hot. The problem appears to be completely random - the system has been running great for about 2 years.
Is this definitely a power supply problem?
AMD Phenom II X4 970 3.5 Ghz
GeForce GTX 560 Ti
Asus M4A77T USB3 mobo
Apologies, it's a XION Extreme Gaming Series 850w PSU, four 12v rails at 18A, 18A, 20A, and 22A. Crap brand, I know, it's probably this that's causing the problem. I've just never seen this specific symptom.
That is the main indication of a too small or failing PSU. When it can't maintain the correct voltage, it shuts down to avoid damage to components. I had a low quality 850W PSU do the same thing. No one can be 100% certain diagnosing from afar, but that would be mine.
That is the main indication of a too small or failing PSU.
In most cases, PSUs are not "too small" because their stated power should technically be sufficient - the majority of non/mild-overclocker non-ubergamer rigs require less than 300W. The usual problem there is that they are poorly designed and fail to deliver their stated capacity for whatever reason(s). Such a rig may work fine on a quality PSU rated for 300W continuous output power but fail with a cheap 650W PSU due to excessive noise, input/output ripple, undershoot, overshoot, etc.
Have you tried running a stress test (run Prime95 and Furmark at the same time) to see if you get the same shutdowns? If it's the PSU, then any high load situation should result in the same shutdowns. You're right that the PSU is not good quality...that's putting it nicely. I'd upgrade to a good quality PSU either way for the reasons mentioned by Invalid.