intermittant fault on ravenheat csi85at?

on this combi when switched to hot water only boiler does not fire at all.on winter /summer boiler fires on hot water but only when heating called. turn on the heating sometimes the boiler fire's but will go to lockout at varying times and some times straight to lock out after 5 to 10 seconds.i have fitted a new thermistor to hot water side and new air pressure switch still have same problem spoke to ravenheat they said requires new pcb. anyone had the same problem thanks