Keston C25 won't run at full power

[TonyMJ]

My C25 starts OK and winds itself up to full power. However after a few minutes it suddenly shuts down (instead of modulating down gradually). No error lights are shown. After a few minutes the boiler starts again, but runs for a shorter time before shutting down.
I can get it to run by turning the boiler thermostat down so it doesn't spend much time at full power.
I've just replaced the exhaust tube with the newer type, but I don't see why this should make a difference.
The condensate trap seems to be clear.

 

[gasmarc]

it sounds like a heat issue but seems odd you are not getting any fault lights, could it be a pump problem perhaps ,i might try wiring the pump direct and see if that makes a difference or turned the speed up perhaps, good luck