Over the past year or so my Tach in my '85 IROC has been slowly changing readings, as of now it is 500-600 RPM off from what it was new at any given speed. The Idle speed seems correct, however. I have changed the ignition module (about a year ago), had the transmission rebuild (3 weeks ago), checked the TCC engagement/disengagement while driving, also have replaced the engine. Does anyone have any ideas what else to check before I replace the tach itself?

~M~