When set to any heat setting, motor starts, glow igniter comes on, gas comes on, but after less than a minute, the dryer shuts off (and display timer goes to zero). Tracing the voltage through I see that the Heater Relay is coming on initially, thus starting the gas, but then shuts off and disconnects power to the gas valves; after that the whole dryer shuts off.

Is the problem the Power Board? or perhaps could it be one of two Thermistors that are plugged into the Power Board? Is there anyway to test them before just buying new ones, like a resistance vs temperature chart or something?

Please note: This problem has actually changed since I started working on the dryer. Initially, only the gas would shutoff after a minute, but the motor would keep running and THEN the burner would repeatedly cycle through the sequence of re-lighting and then shutting-off again.
So, I replaced the two gas valve coils. Now, the problem has gotten worse . . . . as described above.
Please help. I hoping it's not the main Power Board, I bet that's expensive.

Thank you

Bear