My boiler is working (it's heating the water that goes through it),
but something isn't right with it because quite often I can hear
the main gas valve turn off, and then on again about half a second
later, while the boiler is running.

It's due for a service, so hopefully the engineer will be able to
work out what's wrong when he comes, but I was hoping I might get an
idea of what to expect from the knowledgeable folks in this group.
All my attempts at Googling for info on the problem have only turned
up other kinds of problems, not mine.

By "quite often", I mean that it typically happens once or twice in
the time between the boiler starting up and shutting off. They are
usually isolated occurrences, but occasionally I've heard it happen
two or three times in a row with a few seconds in between.

The flue fan stays running, and I think the pilot light is staying
lit, as I don't hear the igniter spark, although I could be wrong if
the spark happens at the same time as the valve turning off (the
sound would be masked by the solenoid click). I tried watching the
pilot light through the inspection window for several minutes until
the boiler shut off, but as luck would have it the problem didn't
occur while I was watching.

The boiler is a Potterton of some sort, rebadged by British Gas as
a "Celsia F1", and is 15 years old. It had the main PCB replaced
about three years ago, but other than that has not had any faults.

Thanks in advance for any insights.

--
Geoff Clare