Can this mean some problem you see when the car is ideling the vacuum reading is at 20hgh but the needle shakes badly what could this be?

 

That is too much vacuum to be a blown motor, but low vacuum and a shaky needle is what you'd see if you lost a seal. However, as I said you don't have a blown seal if you are pulling 20 in Hg (inches of mercury -- the units for the vacuum scale). So don't fret -- your car is fine.

However, I generally think vacuum readings are not comparable from car to car. It changes substantially with altitiude, RPM, and gauge accuracy so it isn't very useful for comparison between cars. Idle vacuum can be useful monitoring the condition of a single car, though. But even then there is a lot of variance. My car pulls 14 in Hg where I live (sea level, with a ported AND damaged motor), but it was only pulling 11 in Hg in the mountains today (5200 feet) on my gauge.

-Max

 

Do you have a mechanical or electrical boost gauge?? If electrical, there may be an issue with noise if the wiring is too close to the ignition wires/coils...
-MP

 

Its a mechanical boost gauge