worcester 28cdi fault?

i've got the a 28cdi and it's causing problems. when running on heating it can stay on all the time, but when the hot water is run after a short spell the hot water light comes on and flashes very fast. the boiler then has to be reset.(not with reset button, but turned off and then on).
i've spoken to the technical line at worcester and they said it was the fan or the air pressure switch.
fan was changed not too long ago and looks to be running fine. i replaced the air pressure switch but it's still the same.....i've checked the hoses to the switch too...........
i'm thinking i may have to change pcb, but i don't want to do that unless i'm 100% sure that's what it is..
any ideas