Factory Formula S Tachometer question

Might be varying battery (alternator) voltage. Put a voltmeter as close as you can to the tach power supply terminal and see if it matches the "swing."

The box you describe indicates some form of aftermarket tach circuit. It may or may not be a good design.

Unfortunately it could be the meter itself Some of those (like the marine ones) had very "floppy" meter movements.

Also, see if the tach meter head is suffering from static electricity. To see that, turn off the key, and gently run your finger across areas of the face. Static will change as you do so and the needle will move. There are various "anti static" chemicals you can find