Hi All,
Our oven doesn't maintain temperature and preheats inconsistently, sometimes it does, sometimes not. I checked the thermo sensor, around 1,000 ohms, and replaced the relay control board with EBR52349704 which looked a little different than the one that was in there. In any case the results did not change and what happened is consistent with what had been happening with the old control board.

Plugged oven back in
Set Clock
pressed Bake, then start to preheat to 350
Oven preheated to 350, chime rang out - Success!
pressed Clear
Pressed Bake, entered 400, pressed start
Preheat indicated a temperature of 350
Oven heated to 400, chime rang out - Success!
Pressed clear, waited about 20 minutes
Pressed Bake, entered 400
Preheat indicated 255
After about 20 min, still said 255 - Fail!
Pressed clear, then Bake, then start to preheat to 350
Display indicated 215.
Oven would not start. This pattern of inconsistent starting and inaccurate temperature reading (seems to be when I set a new temperature once it's started or clear it and try again.)
Unplugged oven. Removed replacement board, placed old one back in. Plugged oven back in.
Set clock. Pressed Bake then start to preheat to 350.
After 5 min received F11 Heating Error.
Pressed clear.
Pressed Bake then start to preheat to 350. Oven reached 350, chime rang out - Success!
Pressed Clear then Bake then start to preheat to 350. Chime rang out indicated a consistent temperature reading - Success!
Pressed clear, then Bake, entered 400, then start. Preheat temperature indicated 350. Temperature reached 400, Chime rang out Success!

Any suggestion as to the culprit before I call a technician?