tachometer ?

I think it would probably be a wire in the steering column leading to a wheel speed sensor or possibly another sensor under the hood

the tach is run right from your coil

try Replacing the wire, or it could be a crossed wire. check it out.

YOUR PICK UP IN THE DIST SENDS A SIGNAL TO PCM AND THEN TO CLUSTER BUT SOME EARLY DODGE PRODUCTS HAVE A SMALL CIRCUIT BOARD ATTACHED TO CLUSTER CALLED A TACH MODULE WHICH IS MORE LIKELY THAN A WIRE PROBLEM. GOOD LUCK

The issue of an erratic tachometer is caused by two things; the most common is the printed circuit board on the rear of the instrument cluster, behind the tachometer. Age takes its toll on the electronics and the solder points become brittle and crack with age, causing a faulty connection and a tachometer that works sometimes, and gives inaccurate readings. Removing the cluster and re-flowing the solder points with a soldering iron will rectify the problem. The other potential issue is the aforementioned hall effect pick up sensor located in the distributor, under the cap and rotor. It receives its sync signal from a metal reluctor wheel is attached to the distributor shaft and spins. Shown in this picture: http://www.turbododge.info/22manual/Chrysler_2.2_2.5_MegaSquirt_Manual/Chrysler_2.2_2.5_mode_on_the_MegaSquirt_files/shapeimage_2.png These metal 'teeth' spin by the magnetic sensor creating a sine wave signal, which the PCM in turn reads and addresses. The four plastic 'blobs' that hold down the wheel often break and cause the reluctor wheel to flop around and give an inaccurate signal (sometimes even flopping around enough to physically hit the sensor!). This too can cause issues. Lastly, double check the connection at the negative side of the coil; ensure it is tight and clean.

also remember the most simple thing. this could just be a bad ground.