Hmmm, the GPU fan stopping is concerning. You might want to try swapping it out, if your fan is burning out, your cpu could be reducing performance to keep from overheating.
How are your heat levels? Obviously the fan is still working in some capacity so it's probably not the fan, but would be good to check just in case.

I had a similar issue with Ram Sticks going bad. It slowly got worse, until 2 of my sticks were getting a lot of hard faults. There is a way to check this in Windows.

Those are the first 2 things I would check. If both of those look good, I would move on to the graphics card, see if you can monitor its performance/heat levels/ect.
If the fan isn't working properly then it could be overheating as well.

As for software, are you running NVIDIA or AMD graphics card? If AMD, check your version on Catalyst Control Center, may need to do a complete uninstall reinstall. There have been some pretty big upgrades for CCC lately.

These are my best guesses as to the culprit, I don't believe it has to do with your internet connection, since it is affecting your fps, and you are loading well in crowded areas.
Good luck