hotwater will not go on?

My 24I boiler heating works perfectly. 2weeks ago I noticed my bath water wouldnt heat properly. After Investigation I noticed that the water burner was igniting for about 5 seconds then going out for 8-10 secs. I purchased a new water flow switch last year which was different from original model but was a genuine worcester replacement one. I have replaced both thermistors but not a lot of difference and replaced motherboard last year. Everything checks according to manual. I am totally lost now Warm but Dirty..lol