|Please note that whenever you remove a heat sink: "I have removed and reseated the heatsink", you MUST remove all old thermal compound and reapply the correct amount of new thermal compound. THAT IS your current problem. Please look:|
Any quality brand will do just fine, you do not need an expensive one, just the exact correct amount applied in the correct way (not too much, not too little, not spread around which creates air pockets).
To undo overclock settings, just reset your BIOS to defaults, then recheck your settings (CPU, memory, etc), time/date, hard drive boot, and boot order. If you are sure that your OC is not extreme, wait until you reset the heat sink correctly and see if that is enough.
You have to be a little bit crazy to keep you from going insane.