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I second what Mark has said. Those wires get a lot of wear, and it doesn't take much for them to start giving erratic readings. Checking the wires all the way from the magneto through the firewall can save you a lot of money. Mine was worn at one of the connectors or at the mag (can't remember which it was now), and a simple repair by the mechanics made a great deal of difference. As he said, much cheaper than messing around with the tach as an instrument.
And to check, you can always swith the leads feeding the tachs, and see if it still bounces on the other one; this is a temporary way to see if the instrument is OK. If it looks the same on the ohter tach which read fine before you swithced, then you know it's not the instrument.
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