Getting specs out of the way:
MSI R9 290x 4GB
16 GB Ram
i7-4790K 4 core 4GHz
Corsair CS550M PSU
Dual monitor 1-3840x2160 @30Hertz
Boring more info: CPU tends to stay 35-38C on average across the cores, idle or brief time in game. Motherboard is the same. GPU idles at 68C, jumps to ~95C by the time I get to the menu in something like Borderlands: Pre-Sequel. I got it down to about 60C before by forcing fans to stay at 100% and slightly turning down memory clock and such. Did furmark GPU test, no crash. 95C almost instantly. Did several tests from 10 seconds to just over a minute. Did a HeavyLoad CPU test. 1:25, CPU maxed at 68-72C across the cores. Dropped back to 40 seemingly instantly upon completion. Also no crash. Have done a memory diagnostic on startup, no issues there. Checked event viewer. Only thing I've found is something telling me the previous shut down wasn't expected.
I had previously been able to play more system intensive games such as GTA5. It would blow out noticeably warm, almost hot air but with something like Skyrim it barely reaches what I'd call warm.
Each crash is exactly the same. It's like the system instantly shuts off. No lights, no fans, no beeps, nothing to monitors, keyboard and mouse lights go off.
I didn't see any problems until a few days ago after I started playing Skyrim w/ mods. I thought it was mods at first because of the timing until I crashed in another game. Outside of crashing everything appears to run just fine, no FPS drops, no graphic problems, no warnings, etc., etc. I've had some Skyrim CTDs but with no warning with those I expect those are mod related. Figure I should mention them though because of the timing. The amount of time in game until I crash varies wildly. When I first tested Borderlands: PS it crashed before I could even get past the main menu. Other times I've been able to play Skyrim for 4-6 hours before I eat a shut down.
Right now I'm trying to eliminate all options that I can before I end up spending any money. My current thought is that the thermal and/or fan setup for the GPU is failing (no idea for how long or how much it contributes to crashing) and the PSU is now failing the system's power needs during gaming. If that were true, would I need to change up the fans on the GPU or would changing the thermal paste be enough?