I second getting an oven thermometer - I think mine cost less than $5, and I found out my oven runs consistently 25 degrees hot. So I turn it down 25 degrees lower than the recipe says, and everything is fine. Now, if you find your oven is not off by any consistent number, or that it's off by a huge margin, then I'd call the repair guy or look into buying a new one. Depends on your budget, I guess.