How do I reset BIOS? I set to defaults already.
If you can't find a reset button or jumper, just pull the power cord out of the power supply, remove the little watch-battery, then reinsert it after about a minute or so, then reconnect the power cord and restart.
As a temporary band-aid, enable C & Q as well as all the C-states. It should at least lower voltage when it downclocks.