Computer displaying artefacts, shutting down on heavy use, erratic behavior on renders. Could it be the PSU or is my GPU fried?

Hello r/TechSupport!, I do a lot of 3D rendering and image editing, and also a lot of gaming; these past months, my computer has been having strange behaviors while doing all these activities and even, sometimes, while simply browsing.

Symptoms :

* Displaying artefacts, like colored dots, bad rendering of some textures in game, black patches flickering on screen. These artefacts/issues aren't constant, sometimes they are there, sometimes they aren't. But they show up while playing graphically intensive games.

* Stutters and freezes; I get these all over, sometimes causing crashes during gaming or 3D editing; I thought it was a hard drive write/read bottleneck so I switched one of my games to the SSD for testing, but to no avail ; I'm still getting stuttering to crash. Only playing the game on very low seems to fix this.

* Hard resets: the computer abruptly turns off during games. the Witcher 3 and Deceit have had these, but not during PUBG or a game like Dishonored 2.

*During work on Autocad; the program's fisplay will often freeze, I can manage to save using shortcuts at that point but getting the display back always means restarting autocad. Don't know if this is due to my rig or if it's an autocad bug though.

Side note: My GPU has two fans on it; one failed at one point and wasn't spinning anymore and making terrible noise; I replaced that fan with another I had that has the same voltage and wattage. I monitor my temps and am usually within reasonable ranges when doing heavy computing/gaming. (never above 80°C)

Other side note; but I think this might be relevant, I built this PC in an [old case](https://www.cnet.com/products/hp-workstation-xw8200-xeon-3-2ghz-1gb-ram-160gb-hdd-xp-pro/specs/), I'm only using the PSU from this case which is rated at 600W of power, but the case has to be from somewhere around 2004... It's the only part in the computer I've never changed or had to change until today. [I've calculated my needed power using this calculator](https://outervision.com/b/tCR2l0), I seem to be within range, so I don't know what to think.

Any help greatly appreciated into diagnosing what may be the culprit.

my specs:

* Windows 10 Home (x64) Version 1703 * Board: Z170A GAMING M5 (MS-7977) 1.0 * 3,20 gigahertz Intel Core i5-6500 * NVIDIA ZOTAC GeForce GTX 970 * 24gigs Usable Installed Memory * Physical drive (250,06 GB) - Physical drive (2000,40 GB) - SSD (300,07 GB) - Two 110 mm fans for the processor