Will go thru start up sequence, but burner won't stay on. Flame light on control board comes on with burner. The light turns off and on irradically and the gas valve closes. Only error code is lockout after a few cycles of this problem. Gas pressure is 7" WC inlet and 3.5" WC manifold. Unit is labeled for nat. gas and has #29 orifice as stated on unit. Polarity is good and unit has good ground. WHAT IS GOING ON WITH IT???