Vokera compact combi - pressure gauge sticking?

I have a small vokera gas compact combi boiler which does not light up every time (works 3 out of 5 times) on the timer setting. Due to having pump problems in the past I have done away with this several years ago and have a filling loop installed which works perfectly. The problem is that the pressure gauge has no reading whatsoever in the morning and I then have to top up to 1.5 bar or just below. At this point it lights no problem then after a few seconds the pressure rises to 2.5 - 3 bar at this point I then have to bleed the system to get it back down to 1.5 bar which is a 30 second operation I would much rather not have to do. I understand why the boiler is not lighting and I am more than capable of getting it to light. Why is the pressure gauge sticking and is there a fix for this? I am not gas safe registered and would not just jump into a fix on my own. I am used to working on gas systems at 200 bar+ offshore as an Instrument Tech so more than understand the implications of amateurs like me working on domestic systems unlicensed I just want to know why it is. Cheers