A Jones tach, as far as quality goes, is a top of the line piece. Mechanical tachs can bounce around some but are more accurate than an electric tach, especially as the RPMs climb. It's hard to say what the exact issue might be with your tach but it would seem to be that something is not adequately lubricated. If the tach is 40+ years old and never been touched, maybe it ought to be looked at.
