Why would a C150's tachometer fluctuate?
It has a little internal generator that creates a voltage to drive the needle.  The generator, and a gearbox that runs the hour meter, is spun by a flex shaft from the engine.  You can have problems stemming from the flexshaft binding and winding up/releasing, but there is probably an electrical issue with the internal generator or the circuit that drives the pointer.  A mechanic might swap out the flex shaft and see if it still does it, and if yes, swap out the tach.  Or vise versa (If I had an other tach handy, I'd disconnect the flexshaft at the tach and work it out from under the panel and connect it to my spare tach and try it out).

In any case, if it's a Cessna 150, and the tach's hour meter shows the airplane's original hours, no doubt it's 43 or more years old and is toast.