Just a thought, probably will not fix anything. But where is your page file? Try disabling it or moving it to another hdd that the games are not on.
Something tells me at this point. Its ether the games are poorly coded, modded or warez, something (likely the GPU or motherboard) is bad and throttling the CPU or GPU.
There is a tweak or setting that was over looked.
No AV? are you sure there is no malware in the system? Maybe format all you hdds and start clean, do not reuse any files just to be safe.
Do you have another system you can test he video card in?
1. I tried to disable page file — no changes.
2. No AV or viruses. Checked with multiple scanners.
3. Nope. There isn't any rig which can handle my 1080 Ti. I only can test weak AMD GPU in my rig what I want to try tomorrow.