fx8320 (or any vishera CPU) have this bug in their temp reports giving lower than ambient temps, like the 10-15ºc you mention you had, those aren't real values due to this bug.
In order to have precise temp readings you need to use AMD Overdrive, not even HWMonitor /HWInfo etc will be able to accurately read until AMD releases some hot fix for this (if they'll ever do that)
As Memhorder said, 42ºC is not a bad temp, could be better with your cooler though, perhaps thermal paste wasn't properly applied / bubbles formed in between while reseating your cooler as you mentioned in the beginning of your post?
Right now the only issue in your system seems to be that 80%+ usage of CPU, so find out which process is the culprit.