Tachometer reads low or oscillates when battery is near full charge or alternate charging source is driving battery voltage higher than regulator set-point voltage.
The alternator tachometer signal level is very low or not present when the voltage regulator is not demanding current (amps) from the alternator. In this condition, the field current is near zero so the field is not magnetized and does not induce a signal into the stator windings. This can occur when a multi-stage regulator switches from bulk charge to float charge and there are little to no loads on the battery, or when an alternate charging source such as solar, AC, or other engine alternator is on. When battery voltage is at or above the regulator's set-point voltage, it will provide little or no field current.
Add resistor to supply minimal alternator field current. The resistor value should be selected such that it provides sufficient field current to induce a good tachometer signal while not overcharging the battery. The resistor values will likely be in the range of 100-1000 ohms.
A small 12v light bulb can be used, as it's resistance is near this range when filament is hot. (Cold resistance is much lower.)
Where to connect the resistor depends on the regulator type or configuration. Some control field current by sourcing a positive current (other side of field winding connected to battery -). Connect one side of the resistor to the field control signal out of the regulator and the other to ground if regulator sources negative field current, otherwise, to switched battery +. If you don't know which it is, try one or the other.
Verify that the resistor value selected provides sufficient tachometer signal at low RPM while not overcharging at high RPM.
Turn on battery loads such as lights until tachometer operation is restored. This is sometimes practical depending on regulator and alternate charging source characteristics.
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