Ferroli 38 HC/Heat Exchanger?

DHW Run's hot and then goes cold?
I have a Ferroli Opimax HE 38C 4 years old,
When I turn ON DHW run's hot and then goes cold
A03 come up, sensor fault ,I have been told that the fact i can reset the boiler and it will start up again and then run hot again and then goes cold that the sensor is operating correct.Could it be the pump not pushing water through at right rate,any idea,Its been said that it could be a blocked Heat exchanger.The Rads seem to also take a while to heat and some dont heat at all have bleed all air out of system.They maybe just need to be balanced.If I drain the whole system and refill would this clear any blockages.I am in Dublin and local ferroli engineer has be out a number off times,trying for the last two week to get him back,anyone know anyone in Dublin that knows about theses boilers,