I have a gas oven with a convection fan and when it gets up to temp, the light goes off. This is with the convection fan running. If I look at the portable thermometer in the oven, it is 25-50 degrees below target. The oven is only a year old so I think it is functioning correctly, but my wife is mad because all of her recipes are undercooked and hard to control. What is the proper way to manage an oven like this? Would longer pre-heat times help it be more consistant? For the record, we open it as little as possible to keep the heat in.