'98 Speedometer not smooth, jumps every 5 mph

Just bought a '98 cargo astro with 130,000 miles, and it runs superb. The only issue is that the speedometer is not smooth. it gives a pretty accurate reading when accelerating and cruising, except that it jumps every 5mph...however when braking its pretty off, also going down it skips about every 5 mph...any ideas?

Check the VSS, (Vehicle Speed Sensor) on a capable scan tool. Watch for smooth operation, (no spikes) while accelerating and cruising. If you don't have access to a scan tool, check the connection at the sensor, (located on the tail housing of the transmission), it might be loose or corroded or the VSS might be faulty.

thanks alot :)