Pull your circuit board and check all of your solder connection with a magnifying glass and a good light. All of mine were cracked where the harness plugs onto the board. I think it is from being mounted so close to the power vented flue expanding & contracting with heat.
If not... The temp sensors could be holding it out. the diverter plate is known to fall off this unit causing the water to overheat.
if not..The igniter acts as a thermocouple, sensing the flame & telling the gas valve to stay open & the spark to shut off. that may require replacement.

Happen to me too Mark. Need to replace the circutboard.
DAVE.