Can someone please shine a bit of light on a problem i'm having with this turd boiler. When hot water is called for, it lights for about 30 secs before going out briefly then relighting itself then doing the same thing over and over so the water runs hot, cold, hot, cold. I've only been a gas engineer for 2 years and i've come across this problem many, many times with other boilers but i'm a bit stumped on this one because i have done the obvious things like cleaned the plate heat exchanger, changed the dhw thermistor, checked the condensate isn't froze, removed the wires from the condensate trap to see if it does the same thing (which it does), checked the wires and leads for continuity and changed the flame sensing electrode but it's still giving me grief. I don't want to start throwing parts at it so any help will be much appreciated.