have been working on an old DUKE oven for the last 3 days intermittent problem. custome wants the unit at 500 degrees, unit goes ovr to 550 and then shuts off and does not recover till 200 degrees i changed the t-stat, and the off/cooldown/oven on switch. still the same problem. question, does the unit have a hi limit switch? and if it does where is it? im also think maybe gas valve could be sticking. driven me crazy!!!!