I have a Lincoln conveyor oven model 1116-000-U-K1837 which is cutting out on temp. way before the desired set point temp. The thermocouple probe, control board, Hi limit t-stat, and display board have been replaced. Set point is set to 550 degree F and temp. cycles between 400 and 450 degrees F only. This is on natural gas. Anything else I need to be checking on this unit?