If you turn off the right hand service valve off this would stop anything passing down the heating if you then have hot water you could have a problem with the diverter valve but it is unusual on that model.once you have checked that you need to check the gas burner pressures min 1.5 max 7 the book says 5 but trust me,the chances are the gas pressure is incorect .if that is ok then I would be looking at the domestic hot water heat exchanger not transfering the heat this would allow the boiler to run very hot and sometimes shut down for short periods before cutting back in again,Im presuming all other taps are the same?and that the problem is not with the shower passing cold water into the hot which will blend and give the same effect its difficult with limmited information and also not being there hope this helps
Dave
Dave