problem with biasi 24s boiler combi?

my boiler let me run hot water off for awhile then it would go on lock out and red light come on, press to reset then sometimes it would run for longer or shorter periods, plumber came ,said hed cranked up air pressure switch in which it ran fine for couple of weeks, now same problem again, also same when heating is on , runs awhile then locks out. Plumber said get new a p switch , i fitted new one correctly but when re assembled it would not fire up, it went through the lighting sequence but would not light . I would appreciate any ideas .Thanks