Usually first thing in the morning when the heating first comes on....it appears to be some sort of cutout (perhaps a safety feature) which is intermitantly flickering on and off very rapidly in spasms causing the pressure guage to fluctuate down and then back to normal.
If I run the hot tap whilst it is doing this it will normally stop it doing it and the water that initially comes out the tap is VERY hot....and it will normally be fine then.
It is a mystery and quite annoying even though the system is still basically working.

the model of boiler is a "Vokera compact"

Karen Webster, April 2008

The most likely cause of this (pressure fluctuation) is due to the expansion vessel inside your boiler being low on air pressure. This is a two part device - one side has air pressure at around 10 psi, the other side has water at usual pressure, and the two are separated by a rubber diaphragm. Perhaps you'd like a heating engineer to check this for you? Failing that, if you have a car tyre pressure gauge and foot pump, you can check the air pressure inside the vessel and adjust it accordingly, using the tyre type valve thats there to see. If you press the valve with a matchstick or something though to test and water appears, the diaphragm has packed up.

John, April 2008