AEG Competence Main Oven heating 3210BU

So I recently posted about my oven seeming to burn items at the edge. Its 15 years old now but has been good. I fitted three new thermostats on the main oven which are a newer model (ET51). To my knowledge I have wired them up all correctly. The problem comes with the new ones just dont seem to get to temperature. I have used an oven thermometer and I know they are not accurate but you could tell because food would take 10 to 15 mins longer than the makers instructions. Ive had to put the old thermostat back in now but thats about 50 degrees out now (probably me messing about removing and fitting). I cant see anything else in the circuit causing the oven to wander. The door seal is new. I am loathe to replace the oven but its looks like thats my only option.