You could try unhooking the battery for a few minutes and see if it fixes it. It's probably a stepper motor. They are used on the instrument cluster and go bad. I believe you can find them on ebay for less than $20. Not sure how much of a pain to replace, but I don't think it's too bad. If the battery thing doesnt fix it, search the forums for stepper motor and you should find some info on how to do it and how much.
chevy runs deep, Don't step in chevy!
67 Coupe 289 - TCI C4 w/2k stall; Holley 4150 570cfm; Performer intake; mild cam; Pertronix HEI; 302 heads; Roller Tips; Hedman Tri-Y headers; H-pipe w/Flowmaster 40's; 9 in posi 3.50; 205/50-17F; 225/50-17R.