Vokera Maxin 24e & 28e. domestic hot water problem?

The domestic hot water is not working at a constant temp or may not work at all, wheather the heating is on or off. The heating works ok but when the call for domestic water is needed the burners switch on and off at 20 second-ish intervals. this is the same when no heating is calling. New diaphragm fitted, pressure vessel at 1 bar, can throw water out of vent, pressure increases when system running.