I have a Ducane furnace, model MPGA075B5. It's about 10 years old. My blower on it started running intermitantly, on 90 sec off 3 minutes or more, when I turned on my AC this year. Changed the capacitor with no improvement. The repair man said my control board was bad. I let the blower run like it was to get some cool in the house until I located a new board. The blower eventually stopped working. I've just replaced the board, one wire at a time, and still nothing. I've considered a new thermostat but my AC turns on and runs and my furnace itself turns on too but no blower in either case. The cage turns freely. Any idea's ?

Thank you