I can at least answer why reseating the cpu was an early response. Heat is one of the few things that would persist across full reboots and gradual heat buildup

I can at least answer why reseating the cpu was an early response. Heat is one of the few things that would persist across full reboots and gradual heat buildup could easily cause these symptoms. There are a lot of places to check still if that's all you have to go on but the cpu is a decent first choice as it's not uncommon for the thermal past to be applied poorly.