You're a gamer...

I haven't seen a gamer yet, select other than optimum setting to run anything. What happens is, if a PC isn't "cooled" enough it finds the weakest link. On the whole, the PC takes a licking on "heat stress" and starts to degrade. One item that can succumb yet allow operation or appear so is the "video" side of things. It will simply not display even though it is apparently up and running. -OR- you lost one leg of voltage from the PSU which isn't 12V because you have fans running. One way or another as i stated it finds the weakest link. In that you already mentioned a previous replaced PSU, it may have moved beyond that and gotten the already heat stressed remaining component(s), either video or mtrbd. and got hosed. The sad fact is, other than replacing parts and reducing the PC to a basic one will you be able to determine this or that. Even if you believe you have adequate cooling, it needs more or better.

As for video cooling, sometimes direct cooling on its area of operation helps greatly as it is one of biggest heat sources in a PC. test it in another PC and check results. Closely inspect the PC for any obvious damage, check for "bad caps", yours should have been build with "mil specs" or more robust metallic based caps. While these don't blow they do fail still, use proper testing tools.

tada -----Willy happy