the LACK of LUBE or Graphte on the speed0 cable IF it's not an electric sending unit type. The speedo cable spins inside another cable. Moisture will cause rust after a few years and this will cause the speedo cable to start to drag and release. If you remove the cable from the back of the instrument cluster and squirt in some graphite, this should help with the bouncing.
I had the same issue with my 93 ford tempo, it turned out to be a governer gear in the tansmission that would cause it to spike the speedometer. If you get it looked at quick you will save lots of cash.
Possibly your speed sensor. My speedometer would bounce around & then soon after it felt like my transmission was slipping. Turns out the speed sensor controls the upshift in your transmission.
got the same exact problem.
sounds like a broken speedometer cable.
If it's an electronic (analog) speedometer needle, you probably have a problem with the dasboard grounding.
The gears in the speedometer are stripped. If the odometer is working then you have input.
TCM Malfunction, And/Or Speedometer Sensor Malfunction.
i would have to bet on the speed sensor
If it is the same problem that appeared on my '04, it is going to be the instrument cluster. The speedometer is part of it and couldn't be changed independently. Mine started bouncing some when I would first start it up, then start working normal. I ignored that for a while. Then it got to where it would not move at all till I had driven some , then it would start working right. It finally got where it wouldn't move at all or just peg out. I had the cluster changed and that fixed it.
The speedometer on a 1996 Honda Accord EX is the pulse variety. It is likely that the teeth on the speedometer gear are not meshing properly.
If it works then all of a sudden it starts bouncing back and forth its your Vehicle speed sensor. NAPA part number Echlin VSS318 for a 98 Taurus. It's a little tough to access but achievable.
If the speedometer continues to work - probably a broken gear etc. in speedometer head and probably will have to be replaced. If speedometer does not work -- probably a broken speedometer cable