I am trouble shooting a furnace. When there is a demand from the stat, the burner fires long enough to get the blower to turn on then runs for several minutes. the burner turns off (voltage to the gas valve dropsout to zero). After about 15 seconds the burner refires and runs for about 20 seconds then drops out again. This cycles over and over with the burner turning off until it will sometimes just turn off immediately after the ignitor turns off. I suspected the flame sensor, but it made no difference after I cleaned it off gently with a mild paper. With it unplugged, the burner extinguishes immediately after ignition timout (simulating no flame sense) as it should. Limit switches are all in series (3) and have continuity so I don't think it is that either. I have checked the vacuum sensor as well by uplugging it and it seems to be operating as well (wont allow ignition when disconnected). Exhaust blower is running fine. Gas valve seems to be operating fine when it has voltage and only shuts off when voltage drops out. It seems like something is telling the control board to drop the voltage but I can't figure out what it is. Anyone have any ideas?