what car model and year is it? because i had a 1990 honda accord ex.... turns out they have defective speed sensors.. my speedometer needle would bouce up all over the place and then eventually after 2 weeks stopped working all together and then randomly would pop up on the freeway going 75mph.. it would also just jump up and down randomly and almost dance to the beat of the radio up and down etc.... really weird.. but ya turned out my speed sensor was bad so i had it replaced.. kinda expensive.. i think i paid under $150 beecause i didn't want to bother with it at the time the part isn't too expensive but i bet thats what it is..