I have a Payne 394gaw furnace. The house thermostat calls for heat, the igniter assembly sparks and lights fine.

(problem 1): It takes about three minutes for the thermocouple in the igniter assembly to close

(problem 2): The main gas starts, runs for about 10 seconds, then stops, then starts for about 10 seconds, then stops, then starts and runs, and eventually the main fan starts and we get heat. During the starts and stops, I can hear rattling/buzzing inside the main gas valve.

I've had the igniter assembly and the main gas assembly replaced. Do I need a new mainboard? New main gas valve? New igniter assembly? All of the above?

Any suggestions for more accurate diagnosis before I go parts shopping?

Thanks in advance.

-Mike