Back on topic, my boiler used to do the pressure fluctuation thing a lot. Sometimes keeping pressure for a week or more, other times dumping it right down to 0BARG in the space of a day.
I thought it was probably a pressure vessel / diaphragm fault, as I had no leaks anywhere. Put it on my round tuit list.
Doing a spot of kitchen renovation, I removed a radiator. During the removal, it appeared that the feed pipe to the rad was a bit ropey, and the connection valve onto the rad was seized, which was going to rip the pipe out. Drained down, replaced the rad valve and a bit of pipe – and noticed a green staining on the pipework. “Hmmm… ” I thought.
Since re-assembling and refilling etc, the pressure has been steady for a month or so now….
So, tiny weep on a fitting can lead to all sorts of pressure gauge fluctuations, and can be a right pain to find.