Potterton profile 40e boiler. Struggles to ignite.?

Sounds like it has come on for about 5 to 10 seconds, then goes off for a few secs. This sequence repeats itself for a few minutes, then it lights up and works fine. The fan is noisy, but works. I wondered if the problem might be to do with the fan or perhaps the pressure switch, or something else. The boiler is about 10 years old, and has been serviced occasionally, but no parts changed. Thanks.