I have a Goodman GPD125-4 furnace. There's a relay that clicks repeatedly, and the main blower goes on and off. I replaced the flame sensor, and it still does it. I'm thinking it might be the circuit board, but I'd like to get some better opinions before I drop $100+ on it. The furnace eventually turns on and runs fine; it's at startup that it acts up. The secondary blower turns on fine, as does the burner.