Sounds like the thermostat to me as well.
Locate the old one and buy any other one that will fit in the space.
I think some one wants to sell you a new unit.
We had an OLD Roper when we moved into the house - my wife complains its too small and it doesn't hold temperature and blah blah blah. I borrow a thermocouple from work and stick it in - oven is dead nutz on temp.
We did replace it with a much larger stove that had 6 burners and a massive oven and its got a chic hood w/ a heat lamp as well.
If ya wife and you bake a lot then you would appreciate a new stove.
If you don't do that much then just fix it yourself (remember to UNPLUG it before you work on it !!)