spedometer problems

all of the sudden my spedometer(spelling?) started acting up. when i get up to about 80 km/h(50mph) it starts boucing really bad to like 15 above and below my actual speed. this is very recent and is getting on my nerves already. does anybody know what would cause this so i can fix it. not a major problem but it is irritating

What year vehicle? Sounds Vehicle Speed Sensor-ish or speedometer cable-ish...depending on the year.

it is a 92

id say cables broke or gear in t case stripped