Faulty AEG oven

My expensive AEG oven started having a problem with temperature control recently, while still in the warranty period. After many phone calls and weeks of waiting, a part was replaced on their instruction by a sub-contractor. The oven worked fine for two bakes, before burning my bakes again. The small oven thermometer placed in the oven to test the temperature shows variances of as much as 50 degrees. For the last week I have tried without success to get my problem resolved and the promise of 'being contacted by management' has not happened either. As a baker, my oven is my income and this lack of care and service is incredibly frustrating!