I have a 36" Thermador

Exactly what part you replaced.And was there any difference in the problem after and before replacing the part. ------------ as per the problem the possible faulty parts can be Damaged igniter or faulty safety valve. 1. The thermostat control. 2. The igniter. 3. The automatic valve (also called the safety valve).
---------- The thermostat, igniter, and safety valve are wired in a series circuit. • When the thermostat knob is turned to a selected setting, electrical contacts in the thermostat close applying power to the series ignition circuit. • With power applied, the ignitor begins to heat. Electrical resistance in the ignitor will decrease as the temperature of the ignitor increases. • Electrical current in the series circuit increases in proportion to the drop in ignitor resistance. • The igniter limits the electrical current in the circuit to 3.3 to 3.6 amps. The surface temperature of the ignitor will be between 1800° and 2500°F. • When electrical current has reached the proper amperage, the automatic safety valve will open, releasing main burner gas. • The ignitor stays energized while the burner is lit. Once the selected temperature is reached, the thermostat contacts will open and remove power from the ignition circuit. The automatic safety valve will close after a few seconds, and the burner flame will go out. ----------And the burner will relight when temp-reaches down from selected.But if the safety gas valve does not open then it will not relight, or if the igniter is weak then it will not relight.The gas pressure is low then it will not let the cook top to relight.When you turn the selector off and turn on,the whole system works as it gets a flow of current.But its not working automatically as it should.Also other possibility can be selector switch/know itself.---------- This possibilities are to be checked.Keep updated.Thanks.Helpmech.

Gary,
What you are asking for is possible in electric units and not in gas ones. The gas supply may start when the temperature reaches the lower limit of the selected range but who to ignite the gas? This may lead to an accidental fire in the kitchen. No cook top will offer what you need. If it was electric then the circuit breaker/maker would have done the job.

Hello & Welcome to FixYa

The problem is with the ignition device that controls the griddle is bad and needs replacement. Please check this picture for parts that will help you. Please get back to me if you have any other questions.

Kevin