Do you get a 'fault' light (RED light on control panel)?
IF the ECO is tripping (High temp t-stat) the water would be VERY hot (Opens at 180*F) and 'fault' light would come on. Also it would refire after temp cooled down to 110*F (t-stats close---turning ON/OFF swicth OFF resets)
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