is your tank vented and is the vent clear and working? I had a similar problem with my challenger. I used a vented cap but the hole for the vent wasn't drilled all the way so it wasn't working. If you drive your car then bring it home open the gas cap and hear a pssssssss then you found your problem. Just sitting in your driveway if it's still hot where you live should help you do the same test.