We have a 22yr old Vaillant VCW Gas open flued boiler, doing CH & HW

We've recently had it serviced, including having the heat exchanger
fins removed & washed clean of debris. A new thermocoupler was
installed too. It then worked fine for approx 3 weeks.

Now it just shuts itself off, including the pilot. Sometimes this takes
an hour to happen after firing up, sometimes within seconds of firing
up.

This problem happpens 70% of the time. If there is any pattern, the
morning cycle (HW only for an hour) runs just fine but when the CH
comes on, clunk, off it all goes.

You can relight the pilot just fine but, if either CH or HW is on,
clunk off it goes within 30 seconds of it firing up.

I noted a couple of sites on the web suggesting pushing the reset
button (makes no difference) OR a microswitch behind the red button
(not willing to take it apart) OR a blockage or misfunctioning valve
slowing the flow, thus overheating the water (wouldn't know where to
start, though I do note the rads get very hot when working so I don't
think sludge is the answer)

Any wisdom you could impart would be welcome, if only a pointer to
something else I hadn't thought of

Thanking you in anticipation
- David