2 days ago my tachometer started jumping from 3k to 6k everytime I shift (at 2.5k). The weird thing is the engine never actually reach those high rpms.

At cruising speed at (4th or 5th) reving at 3k-3.5k my tach needle begins to jump again but this time I can feel this lag or slight stutter. To avoid this, I make sure I cruise at 2k to 2.5k rpm.

When I turn my aircon (at idle) it also gives this erratic tach reading.

Does this have something to do with the IACV? Where does the tach gets its signal? from the ecu directly or rpm sensor(but where)?

Could it be my ignition system (like voltage regulation) be the culprit?

Any advise is really appreciated.

BTW, I drive a 93 honda civic ex 5spd.