1987 grand am 3.0 problem

well here goes, i hope any one will help me out. my problem is my speedometer needle goes crazy when driving , it jumps up and down,then i notice the motor kinda stutters as this is happening ,but when it stops working(the speedometer needle in dashboard)the motor seems fine as i'm driving.and also when the cars idling it revs up and revs down.please can any1 help me out.thanks to who all answers.runaround