Whirlpool Electric Range oven temperature inaccurate

Approximately 12 year old range, all of a sudden the oven temp is all over the place. I set it at 400 and it might get up to 350. Set it at 450, and it might get to 400, or might not. Tried the "recalibration" suggested in the manual, but that only allows a 30 degree adjustment up or down, not enough to correct the problem. And even after recalibrating, it still didn’t to up by the right number of degrees. What is the most likely issue? Thermostat? (can’t find that in parts list), Temperature Sensor? Control Board? Are there any other tests I could do to pin down the most likely suspect?