Sorry to see that your boiler is on the blink… While I’m an ocean away and our heating systems differ greatly, (even the oil boiler I had was from a different century) I would start by looking at your circulation pump.
As the temperature gets into the operating range and the pressure rises, I would assume that the control unit sees the need to pump out the heated water and bring in cool water so the actual boiler chamber does not overheat. Failing to pump enough would cause an overtemp error condition- thus the shutdown- and, given the fluctuation in pressure and the ominous clunking sounds, I’d put money on a broken impeller in the pump housing.
If I were in country, this would be an excellent excuse for a road trip, but my tools and I are on the wrong side of the pond.
By the by, the website is the wife’s business- I’m adding your WP calendar plug-in to a site and stopped by to get the particulars.
Best of luck- I’d love to hear how close I was to the core issue, should you have the time and inclination…

Regards and thanks for your contributions!
James