Sime Friendly Format 80E - flame cuts out intermittently?

Hi, I hope someone can give me some advice.

My boiler cuts out after it has been on for a while. Initially it ignites perfectly, with both hot-water and also central heating. However, after a while the flame cuts out. The flame will remain out until I turn the boiler off and on again, when it usually re-ignites.

This has been going on for about a year now and it has been getting progressively worse. Now the flame cuts out sooner and it will not re-ignite unless you leave the boiler off for at least an hour.

I have had a plumber look at it, and when he first removed the front cover, and turned the boiler on, the flame ignited and stayed on for ages. Furthermore, after he did this the boiler worked perfectly for about a week. Then it returned to it's old tricks again.

I got him back and he thought that it was the air pressure switch, changing which has had no effect at all. His other idea is that the fan is faulty.

I am skeptical about his suggestion as the fan appears to be blowing out well, and starts up every time, and remains on after the flame cuts out.

Has anyone got any ideas?