How can I fix my Chaffoteaux Britony boiler SE 80?

The hot water, whether the heating is on or off, fluctuates between hot and cold in a 30 odd second cycle. The secondary DHW heat exchanger has been cleaned and the electrical circuit board has been replaced (the very expensive bit). This was replaced because of another fault to a fan relay. The fault has been gradually getting worse with the water temperature varying slightly in the beginning, but now fluctuating between very hot to cold. The burner does cut in and out as if it is trying to regulate the temp. But not at the same rate of the water temp fluctuations. Generally the Heating is behaving itself but on the odd occasion after using the hot water the radiators get very hot all of a sudden. We then turn off the heating to let it settle down, and all returns to normal after.