How can i fix my Vokera Linea 734 boiler?

I have theVokera Linea 735 combi boiler which is 3/4 years old. We keep getting problems with the hot water, the boiler heats the water for about 40 seconds then goes off for 10 seconds back on and off until after a few days we can't get it on for long enough to have a shower. Our plumber has changed the heat exchanger at least twice. In December we had all the radiators power flushed. Early January the heat exchanger taken out and rinsed so no muck was in it and a filter put on the water supply. Today the problem of hot and cold water has started again. Has anybody any answers?