Baxi Solo slow to fire up?

I have a Baxi Solo 3 PFL, just recently the burners have been very slow to fire up after the pump starts, both when the system is on timer and when adjusting the room thermostat; 20 minutes is average. The burners also go out even when the room temperature has not been reached. It does appear that this happens when the water in the water in the hot tank is hot. The system is fitted with a Honeywell 3 port motorised valve which appears to be working OK. Does anyone have any idea what the problem might be. I replaced the PCB and flexible tubes to the fan about a year ago, so I hope its not the PCB again.