HELP!! speedometer problem

hey, i just bought a 92 DX hatch and the speedometer seems to work at random times. i think first the car has to like warm up almost for the speedometer to work. when it first starts moving, it sorta shakes around a little bit, going from 40 to 20 to 35 and so on. then it starts working normally. then it shakes a little bit more. and this is also weird because the temperature and fuel gauge work. so does the mile ticker. and i also don't feel like letting the car warm up for 30 minutes before each time i go out just to make the speedometer work. any suggestions on how to fix it? thanks.

anybody?

did you check the speedometer cable??