we purchased a jenn air double oven range in 2007. the stove is gas and the oven is electric. it can be either convection or regular. it has always been a little off...sometimes cookies bake in the set time, sometimes they take as much as twice as long. could never really be sure how long things would take. even bought a thermometer to check if it was heating properly and it showed the temp was right on the money. just learned to live with that. now what is happening is the cookies burn along the back wall of the oven and the fronts are not so i am getting half raw, half burned cookies. any ideas what to do? thanks so much.