Apparently, i didn't solve shit. The game goes back to sub 20 fps after a while and the only way to fix it is to restart windows. God damn it.
You overclocked. The CPU is probably hitting its thermal threshold and throttling itself on the hardware level.
The thermal threshold on old athlon CPUs is pretty low, you would be surprised what is "safe" for them. You start to run into trouble around 60C, which is considered a cool and breezy temp for GPUs.
In summary, your CPU is really, really old. Get a new one if you want to play modern games without performance issues. (Though Dark Souls 1 IS a bad port, you shouldn't be having issues with games from that era.)