Vokera linea24 hot water problem?

Please help intermittant problems on hot water, code04 flashes for fraction of second -temperature drops, then it corrects itself,green light returns and temp increases and everything ok perhaps for several days. Pressure guage at 1,5bar with no pressure drop.Just been serviced,all measurements ok. Very occasionally code 03 will flash immediateley followed by code 04 then returns to green.Pressure at tap excessive and has to be reduced for hot water. thanks for any feed back.
John