Micron 50ff cutting out before room up to temperature?

Hello all; have an old 50ff although in the last year it has had fitted a new air pressure switch, new fan, new pcb, new thermistor, heat sync and new honeywell controller (st699). So yes, new boiler practically! However, last replacements (pcb/thermistor/heat sync) have not resolved boiler cutting out before temperature reached on room thermostat. Have had engineer re-pressurize loop and confirm bar is good, in & out flow pipes to boiler both hot when it's working. Occasionally 'reset' led lit; have to leave it off for a while to restart so seems to be the boiler pcb cutting out in response to temperature. But never used to happen, although very similar behaviour before last batch of replacement parts fitted. Any clues as to what might cause this cut out? (The pump works fine, single pump system, that gets very hot.)