Fit a larger filter (larger internal volume) the flutter should stop - its the gauge reading the "pulses" from the cylinders - the closer the pick up point is to the inlet manifold - the bigger the "pulses" - if you fit the gauge pick-up-point at the output of the intercooler - you may not need a filter (the filter is required for its "volume"- a sealed can or container can work, too)
if the guage pick-up point is close to the compressor outlet - it can pick up the compressor "surging" etc
Your boost goes up and down in normal use - when you are accelerating hard, going uphill etc, or on "full throttle" the boost goes up, running downhill, "crouising" on the flat, at tickover - the boost comes down - the gauge measures this - it may just be doing its job
are you actually hearing hissing/ air escaping? or a lack of performance?
PS - a mechanical boost gauge does not have solenoids, or electrical parts (apart from the light ) - its a bit like a barometer (its a realatively simple mechanical device) they are fairly reliable, mostly, they are accurate give or take +- 2PSI - you will get a different reading depending where you put your pick-up-point (high on the outlet of the compressor, lower after the intercooler(due to the pressure drop))
The reason you place the pick-up-point after the intercooler is it takes the intercooler pressure-drop into account and gives a more accurate idea of the boost "seen" by the engine - on advanced set-ups there may be 2 boost gauges to measure the pressure drop through the intercooler or measure the output of the compressor to optimise it, etc