Potterton FF 40 Boiler start up problem?

My boiler will try starting up on the timer. The fan starts and the air pressure switch clicks on for 15 secs but then it all shuts down. 3 secs later it will try again but shut off after 2 secs. Will keep doing this and eventually, but not always, start and stay on after about 30 mins. On the advice of a friend, yesterday I used a hairdryer under the APS and it started immediately. Any ideas where the prob lies? Read an earlier thread about replacing capacitors on PCB 407677 which sounds similar but puzzled as to why hairdryer trick seems to make it work! Any help much appreciated!