CO2 regulator trouble

I have a CO2 tank with dual gauge regulator.  I have had problems with it in that the needle gauge vibrates when the valve is opened.  It didn't do that when I first used the system.  Had the regulator repaired but the problem recurred.  I even borrowed a friend's regulator and noticed that at first, it too worked.  Then when I used my airbrush (Iwata HP-CS) with the pressure about 15-20, the pressure started to rise when I didn't have the trigger depressed and the needle would rapidly rise off the scale.  I'd then hear the vibrating and rapid needle oscillations again.  I've stopped using it as I'm afraid I'm going to break my friend's regulator.  Any suggestions?  Is there something I'm missing with the hookups or setup?  Any advice would be appreciatetd.