Intermittent Ignition is usually a ground problem/dirty connections
You should go through and pull all of the wires off any spade connections. Then reconnect them and ensure the connections are tight and corrosion free.
Check the green ground wire connection (from circuit board to frame)
Check the spark electrode bracket mounting screw connection (ground source for spark electrode rod)
Pull connector on circuit board.....clean contact surface with pencil eraser
Doesn't take much (dirty/loose connections) to stop 12V DC
Check the main flame......
It needs to engulf spark electrode. Spark electrode is used as a flame prover.
A milivot signal is generated by main flame and is sent back to circuit board to keep power on gas valve.
Also....need to clean out the combustion/exhaust chamber.
Spider webs/debris will cause blockage and disrupt exhaust causing erratic main flame
