Tach Inconsistant-Jumps & Freezes-'96 DT-140

I have a pair if 1996 DT-140 hp motors. Occasionally the tacks will fluctuate and stick at inaccurate RPM readings. The port side started a couple of years before the starboard side decided to follow suit. I will be cruising at 4200 rpm’s and then it will max at 7000 rpm. On some occasions the tach will jump to zero or some other inaccurate reading, above and below of the true rpm’s. Repeated knuckle tapping on the gauge face will sometimes get it to register correctly, or close to it. I checked all the ground and they seem to be good.
1. Is there an accurate way to check the grounds?
2. Is this a sign of failing gauges?

Any advice is greatly appreciated.

is the pole setting on back of tach
remove tach, turn the dial for pole signal a couple times back and forth
reset it to original setting

justintime,
Thanks for info. I launched my boat two days ago and the tachs worked perfect over the past two days. This resolution will save my knuckles. Thanks.

Good info in this thread, I'd have never thought of the pole switch!

-E

Always