If your 4JH3HTE tach is like the one I have on my 4JH2E, the signal for the tach comes from a magnetic pickup that literally "counts" the teeth on the flywheel going by when the engine
is running. My tach was hunting too. Tapping the front of the tach would sometimes stabilize the reading.
On the rear of the tach there is a range switch that selects a range of teeth on the flywheel to allow the tach to be used with various engine
models. On my tach, merely working the switch through its range with a little squirt of contact cleaner would fix the tach for several weeks.
There is also a calibration pot that gets dirty too. A squirt of contact cleaner and working the pot through its range will restore stable readings. Caution, you must remember or mark the exact location of the pot's position or your tach's calibration will be off.
I finally disassmebled the tach, removed the range switch, and soldered a wire jumper in place of the switch. Problem gone.
If you are not familier with soldering techniques on small printed circuit boards and small components, I would not recommend taking the tach apart.