please help worcester 28 si II combi?

recently had a gas engineer out as stopped working altogether with light flashing approx 1 per sec used reset button it fired up after about 10 sec watching flame through window started to fade away a few clicking noises from control panel and ignition started even though flame had not completely died roared up for 1 sec appx then diminished slightly again only to try again to re ignite ....and so on until flame goes out completely and then light flashes 4 times per sec appx engineer replaced air pressure switch next day worked fine heating and water for appx 20 mins then same again he came back again next day with older colleague four hours later and tech support from bosch helpline still no luck bosch helpline said with the readings being relayed boiler should work fine ....but its not in a last ditch attempt he said problem maybe flu which he stripped re-sealed and re-connected again worked fine for 20 mins although he had to start it off by operating air pressure switch manually and then back to the same once it had heated up been 3 weeks now has anyone got any ideas as it has cost me a fortune in labour already and i am still no wiser or warmer this is as much detail and understanding that i have thanks for reading