How can I fix my Worcester boiler 24si series11?

Hi,I have a red flashing light 5 times a second,boiler will sometimes reset itself,if after 5 attempts it doesn't will trip again,when trying to reset ignition sparking,burners light red light goes off,after a minute flame goes really low then goes out red light comes on.I'm not using boiler have it switched off but if put on start up no problems and will stay on for about 20 mins before red light comes on,I've had a new board fitted think it's called a pcb,was told gas valve and ignition ok still doesn't work also had it serviced,any ideas,thanks.

Holly
October 2012