vaillant VCW GB 242 EH?

My dad's VCW GB 242 EH was leaking at diaphram unit so we replaced it and also the divertor. However, the ignition keeps tripping and when hot water is run from the tap and we eventually get it ignited, it it starts up; then as it's running the flame goes out, then back on again, (the water in the taps goes from scorcing to luke warm), this carries on until hot water is stopped being drawn. All the time, the central heating does work ;-( Sometimes the ignition keeps tripping several times before ignition takes place... any ideas please?