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You don’t give a lot of information…
But by the sound of it I might be looking at :
Quick, Short Fluctuations
A vacuum reading that very quickly drops and rises a couple inches of mercury or psi generally indicates some sort of ignition system malfunction. You might have a misfire as the result of a bad coil or cross firing in the distributor, or you may have a spark plug with too small or large a gap. If the needle is truly spastic, you could have a multi cylinder ignition misfire. On a computer-controlled car, a bad cam or crank position sensor could cause this, but these will manifest in other and more noticeable ways.Wide Fluctuation at Idle
A wide fluctuation at idle generally means that something’s gone awry between two of the engine’s cylinders; one is pulling when it should be pushing, the other may be doing nothing. The only way this is going to happen is if you’ve either blown a head gasket or cranked the head, in which case you’ll see other symptoms. You’re likely to see excessive exhaust smoke that smells like fuel, antifreeze, oil or all three.Valve train Problems
A vacuum gauge needle that vibrates rapidly at idle and then seems to smooth out with rpm can indicate bad valve guides or a bad cam lobe. The bad guides allow the valve to move around a bit while the valves are at their lowest opening point, and the bad lobe will effectively kill that cylinder at low lift. Intermittent drops in vacuum at idle and under cruise could indicate a sticking valve, and needle drop of vibration under acceleration can indicate a bad valve or valve seat. Hydraulic lifter bleed-off will manifest as an intermittent vacuum drop at idle, and incorrect valve lash will mimic the effects of a bad valve.
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