Speedo inop

Got an 84 gmc jimmy 4wd. Speedo used to bounce up and down, but now it doesn't do anything. Any clues.

If I remember correctly, the speedometer is cable driven on that vehicle. If so, when the speedo was bouncing, the cable was binding in the housing and it was in need of lubrication. Without lubrication, it probably destroyed itself. Time for a new cable and housing now. In the future if/when it starts bouncing, remove the instrument cluster, and pull the cable out of the housing. Lube it liberally with lithium grease and reinstall it.

thanx