UNHAPPY WITH GE PROFILE RANGE

My wife and I purchased a complete kitchen of GE profile appliances. We have had a few problems which GE has taken care of with one exception. Our freestanding range does not bake evenly on convection mode, we purchased the JB968SOH2SS convection oven with the hopes of saving time while receiving better baking results, this has not proven to be the case.
After a few visits by a few different service men I was told that they could not make the first range we had work properly,again it had oven problems, they decided to replace the unit, now the second one burns everything on the back of the baking pans and leave the items on the front undone. The oven takes 12 minutes to read 350 on the digital readout but took 40 minutes total until the oven was really at 350 (we had two thermometers in the oven). I was told today by the GE support person that according to them the unit performed as designed and there was nothing else they could do for us. Please keep in mind that our particular range has a suggested retail of $1749.00 .