How can I fix my worcester24cdi problem?

I have a worcester 24CDI combi boiler keep flush the red light need reset pump work and the fan work spark starting low flam comes on and then not completing for the full gas rate, some times the boiler stay on about 20 minuts then start the red light flush, I have changed the PCB and the gas valve but still getting same problem can any one give me an idea please.

Benjamin
November 2012