How to fix a bouncing speedometer on a 1995 ford e 150 van?

Ya, bad cable. Probably got wet somewhere along the line. pull it out, wipe it with some kerosene to clean it. Then coat it with lithium grease and feed it back into the outer cable. When it gets to the bottom, if it doesn't feel like it is all the way in, slowly rotate it as you put light downforce to get it to drop into the transmission socket. reconnect the spedo and it should work. If it doesn't then you will need a complete spedo cable assembly.

No cable. Electronic. Your sender (VSS) is on the rear diff and your problem is one of two issues. 1. You have a damaged tone ring in the differential and it is sending a bad signal. Only problematic between 50 and 70 MPH (80km/h and 120 km/h) 2. Burnt component(s)s or loose connection(s) on PSOM on back of speedometer. manifests as bouncing speedo all speeds and/or no odometer and/or hard shifting transmission.

Interesting.