Vokera Linea 24 No Hot Water on Central Heating Mode?

I am having problems with my Vokera Linea 24, I've had a few engineers to it but they haven't been able to solve it. When the boiler is on central heating and hot water mode when I ask for water the boiler flame ignites for a few seconds then goes off and repeats itself, it also does that when it gets up to temperature on the central heating too, it used to modulate but now just cuts out. The puzzling part is when I turn the selector to hot water only I get perfect hot water, help!?