Question Games crashing after playing for an hour ?

Apr 25, 2020
I've been having this issue for several months now and I've searched everywhere to find a solution to this problem but I can't find anything.
PC Specs:

AS Rock X99 Extreme4
GTX 780Ti (stock)
Intel i7 5820k
Silicon Power 256Gb SSD
3TB Seagate HDD
Thermaltake Smart 750W 80Plus bronze PSU
Windows 10 64-bit OS

First I'd like to say that I've had this PC for 7 years now and the only thing I've upgraded since then was my storage which I did over a year ago. At first I thought it was a simple dust build-up in the GPU fan so I cleaned it and it did not solve the issue. I proceeded to take every part out, clean the entire PC, re-applied the thermal paste on both my CPU and my GPU and this still did not solve the issue, I've tried stress tests (OCCT) and I left it running for 4 hours and it did not crash or say anything wrong with my PC, I also used Furmark and there still was no issue. I'll list the games I play detailing problems if they exist:

Phantasy Star Online 2 (During certain quests it MAY crash, not 100% of the time)
Fortnite (Crashes within an hour of playing)
Yuzu emulator Smash Brothers Ultimate (Crashes within 2-3 hours of playing)
League of Legends (Does not crash)
Brawlhalla (Does not crash)
Among us (Does not crash)
GTA V (Does not crash)
Rocket League (Crashes within 20 minutes)

These are all the games that I've played in recent time and had issues with it crashing, at first I thought it was an issue with games being too demanding and my GPU started to give out but Furmark stressed my GPU more than any of these games ever could, Rocket League is a particularly strange one to me since I don't believe it's more demanding than Fortnite or GTA V. Any suggestions would be great on how I can troubleshoot the issue here, thanks in advance!