It's possible that the heater is the cause of your OH issues- but it's not the 1st thing I would suspect- nor would I go out and purchase one without knowing it's broken to begin with. Remember- a heaters job is to get as hot as possible. Unless there's an unusually high amount of hardness that has accumulated on the elements and actually touches the housing right at the point the thermowell is, or there is so much hardness that is actually stops the circ pump flow, and you have a broken press/sw, I'd look elsewhere.

First and foremost- when you initially see OH, is the heater actually overheating? Quickly touch the heater, is it hot?

If not- you might want to start with new temp sensors...and go from there.

Good luck!