how can i fix my potterton promax 24/2/plus boiler?

my boiler is potterton promax 24/2/he plus, is 5 years old and suddenly stop working.(had no issues in past 5 yrs).plumber replaced the pcb unit.the boiler started to work straight away.however after approx 15hrs it stopped working.i reset the button and it worked again.but it has done the same again.i have once again reset the button and its working .any ideas why it keeps cutting out?