more than likely its the pcb as this is a common fault with this boiler either that or the flame sensing electrode is faulty but my experience tells me its the pcb ive fitted loads of them.
what combi boiler do you have and also mixer taps can become a problem with combis,as they can send a back feed of cold water to the boiler when they begin to go faulty, either that or if your ht water works with a diaghram it may be slightly perished and need replacing, another solution would...