2000 ford windstar speedometer needle goes back and forth

Battery or voltage regulator in the alternator

You'll need to take your dash apart, and replace the speedometer needle. Only do this if the rest of your dash is working. When you replace the needle, make sure you do not push the new needle all the way to the dash, there should be a small space between the dash and the needle. That's exactly what I had to do to my 2002 Windstar.

same thing happens with my 2000 windstar, if you could take the instrument panel out, and unscrew the the back of the instrument panel that covers the circuit board and clean the circuit with a brush, pure bristle brush is best 'coz they're stiff and gets all the dust that accumulated on the circuit board. some dust particles contain conductive material and shorts the circuit board and cause the trouble you have, ever since i did that no more trouble. and to those who are planning of replacing your instrument panel should do this first. it'll save you money. always change your "outside air" filter

Replace the VSS.
use a DTC reader to confirm

If you are working the gas pedal this will happen or when using cruise control, when you let off the gas it comes out of overdrive and when reappling the gas goes back into overdrive this helps cut emissions

you probably need to replace the cable or the speedo

Could be a ciggerette lighter fuse, usually cause this when it blows.

DOES YOUR SPEEDODOMETER HAVE A NEEDLE OR IS IT DIGITAL? IF IT HAS A NEEDLE, THERE STANDS THE POSSIBILITY THAT YOU HAVE A BAD SPEEDO CABLE.

turn signal flasher module or turn signal switch. I usuall try to wiggle teh column switch or move it slowly up and down, back and forth to help determine if it has a bad contact. if i cant get a failure (or turn signal) that way I begin the testing at the base of the column. the flasher module is a common failure on those and isnt unreasonably priced. if i had no diagnostic procedure i would try that first. oh yea, try turning the hazards on and off rapidly several times. the signal goes through the four way switch.

their is a speed sencer on trans might try that first if dont fix problem possible cluster problem