I had the same problem. You have to find a vaccum source strictly for the boost gauge. If it shares with anything else, it will shake. I had mine T'd off my boost controller vaccum source and it shook. I gave it its own source and it's been working fine.
sounds like you have a vac. leak.
I had the same prob. and found that the hard plastic vac. line
going to the gauge had a crack in it. I fixed the crack and fixed the jumpy needle. might not be in the same place as mine but deff. sounds like a leak somewhere.
Not necessarily a leak but there are a number of causes possible. Do you have a boost controller? Those bleed pressure to the wastegate actuator so there is a source for some fluctuation there. Due to the nature of the internal wastegate and its actuator it is unlikely you'll ever have a completely stable boost gauge needle.