System Shutdown During Gameplay

During gameplay (between a half-hour and four hours), the system shuts off. That would imply either an overheating issue or a power problem, yes? I examined my case fans, my cpu, my gpu, and my psu for any problems.

My case is a NZXT Lexa S and comes with the four standard fans: topside 140mm exhaust, rear 120mm exhaust, side 120mm intake, and forward 120mm intake. Nothing is visibly or audibly malfunctioning with the fans, so I imagine they're working.

To check for case airflow, I also removed the side panel and directed a 220mm external fan at the case, but the temperatures remained consistent (according to SpeedFan). Since the fans seem to be functioning and this problem is recent, I think the fans and case are not at fault.

My cpu is an AMD Phenom II X6 1090T BE. I purchased/installed this X6 recently to replace my X4. Given the coinciding timeline between the installation and the system issues, I thought the cpu could be the problem. However, my X4 consumed no less power than this X6, so since my system was sufficient for the X4, it should have enough power for the X6. I also do not believe the cpu is over-heating. For one, the stock cooler, although a bit loud, is new and should sustain standard use (I don't overclock anything). For another, my BIOS is supposed to sound an alarm whenever the cpu overheats, which is never has. SpeedFan supports this with temperatures around 40C at idle. I don't think the cpu is my problem.

My graphics card is an AMD Radeon HD 4890. During gameplay, the card would become hot to the touch, enough that I worried my heatsink/fan combo (which came with the card, as indicated) was worn. However, I recently purchased and installed a new VGA cooler, which did cool the card somewhat better (according to my touch) but not enough to prevent a system shutdown. I suppose my graphics card is still suspect and merely needs an ever greater VGA cooler, but it seems unlikely.

Lastly, my power supply unit is a Xion PowerReal 700w. While I did consider a power supply issue, the power requirements for my computer haven't changed in the two years I've used it, even considering the replaced/upgraded cpu. As such, I don't think the psu lacks the power to support my system. If so, the psu must be overheating, as I've eliminated all other possibilities. However, I wouldn't know how to test this. For one, my hardware setup hasn't changed, so I don't know why the psu would overheat now when it never has before. For another, the psu comes with a 140mm intake fan that, like my case fans, sounds and appears as functional as it ever has. Would I simply need to replace the psu with an alternate and check if the problem persists?

When examining my voltages in BIOS (and SpeedFan confirms, though I know it's not as accurate), I see steady levels: +3.3V @ 3.25V, +5V @ 5.11V, and +12V @ 12.16V. SpeedFan also reports additional voltages: -12V @ -9.48V, -5V @ 1.43V, +5V @ 3.76V, and Vbat @ 3.25V. I know too little about voltages to know if SpeedFan's additional readings are troublesome, if even accurate.

And I've already cleaned and dusted each fan and everywhere in my case.

The only oddity I've noticed but can't place is a high temperature spike according to SpeedFan. At idle, Temp1 is 34C, Temp2 is 40C, and Temp3 is 80C. According to the SpeedFan How-To, Temp3 was termed "Case." Does that mean the air within my case is 80C, even at idle?

Thank you for even reading this payload of information, and thank you especially for any help you are able to offer.