Combi boiler when hot water tap in bathroom is on all is ok ,water is hot,but when using shower overhead drencher the boiler is intermittent on a couple of minutes then off for 15 seconds or so the back on, but when using handheld spray it is on all time albeit the water is not very warm as household taps a very hot.
Is it the boiler or shower please?
The boiler is lit by a diaphragm operated switch. If the switch is faulty (torn diaphragm?) it will need a bigger water flow to operate. Maybe the water flow to the shower is insufficient. See if it works at the tap with just a trickle.