Problem with CPU bottlenecking when I use my PC for more than 3h

Hello, I have a Ryzen 3700X, undervolted to 4.225Ghz and a FUMA 2 cooler (it doesn't go above 72ºC), a RTX 3080 and 32Gb of Ram DDR4 3200Mhz. I play at 1440p Ultrawide.

When I turn on my PC from dead, everything goes perfect, Lies of P gives me 100-120fps.

BUT the issue comes when I browse on Chrome for like 2-3h and then try to game again. It comes down to 30-40fps at max with a lot of stattering. This happens to me with ALL GAMES and even at normal usage. I can't use my PC for more than 1-2 days without restarting it or fully shut it off and on again. I've seen that CPU0 (out of 15) is at 100% when this happens, and all the other threads are chilling. This happens both with the CPU undervolted or at stock settings, I've tried it.

Right now my RAM usage is 32% out of 32Gb, so it's not a RAM problem either.

To be honest, I can't remember since when this happens, but it has been years.

What could be the problem? Should I just upgrade the CPU? But I feel like that's not the problem, for the first hours, it works perfectly. Can be the problem Google Chrome? But even if I close it, the problem persist until I restart the PC.

UPDATE: https://imgur.com/a/r1J0Mo9

This is the problem. Everything is runing on 1 CPU thread. Everything: Chrome, NZXT Cam, Spotify, and a lot of Windows process. Why is everything running in 1 out of 16 threads and how can I solve it?