Is the air speed coming out of the vents proper? If it's too slow it could cause the area around your heat exchanger baffle tube to over heat, thus causing anything (plastic, cabinetry) near by to over heat. A lot of furnace fan motors are capable of 2 speeds (high and low). The manufacturer of the motor does this to save money, thus one motor can work for two different furnaces. It's possible that your fan motor got wired to the lower speed instead of the higher one?? Make sure you have good air flow on the input side and output side. Clogged filter? To many output vents closed off? It would be nice to know the proper cfm