Not likely an erratic tach is the tachometer.
An erratic reading is usually due to a poor connection somewhere. Start the engine and wiggle each wire leading into the tachometer to try to isolate the culprit. Have someone watch the tachometer while you wiggle the wires on the engine side.
Another possible cause of an erratic reading is electrical noise. Does the problem only occur when the cruise control is running or when sending emails via your phone? Or any number of possible electrical interferences. Faulty spark plug wires etc. The problem can sometimes be corrected by putting a resistor in the signal line. It should be placed on the tachometer end of the wire. You may have to experiment with resistor sizes. A 10k ohm, 1/4 watt resistor will often correct the problem without affecting the calibration. If not, try a 1k ohm, 1/4 watt or a 100 ohm, 1/4 watt resistor.