99 Taurus OD OFF Light What Is it?

Wife was driving my car said this light went on (it was green) and the needle on the speedometer died and went to the bottom and would not move for the rest of the trip she was on with my kids.
She gets home and shuts off the car, comes in and tells me about it I go out and look at the car and I see the speedometer is working fine, the odometer is also working fine so is the trip meter? All that is on is the yellow service engine soon light?
Thing is, the last time they were all in the car they said that the car kind of acted weird like it revved funny a couple of time like a car with a standard transmission and someone pushed in the clutch but the speedometer needle went up and down kind of funny a few times while they were on the highway so she pulled over at a rest stop and shut the car off.
When she started it up everything was fine and it has never done that again, this was months ago and it's never done either of these things for me? The OD OFF light is new just a couple of days ago?
Anyone out there have any idea what might be going on here? All fluids are fine and always have been fine, everything gets changed when needed, the car has less than 40k miles on it and is in great shape and has never given me any problems?
These problems have been verified by my 16 year old son? I have not brought this to my mechanic yet because I have not been able to see this happening myself and so far each thing has only happened once? Any Ideas?
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This too much again so soon. But it looks like your Vehicle Speed Sensor is going bad. What engine is in your car? If it is the plain Jane Vulcan 3.0V6 it will be an easy fix. If you have the 24V 3.0V6 DOHC, the repair is more involved and you have your work cut out for you.

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Thank You for your help, I do appreciate it!

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[This followup was posted to alt.autos.ford and a copy was sent to the cited author.]
snipped-for-privacy@hotmail.com says...

The OD off light turning on without you actually disabling OD is probably the transmission computer telling you it detected a failure. The rev up was possibly a downshift into second, which is a fail-safe mode.
Somebody else mentioned the Vehicle Speed Sensor (VSS), which is a good possibility since the speedometer also shut down, assuming it's electronic and not mechanical. If any other gauges shut down, you could have had a power problem to the dash.
If the check engine light turns on, get the code. It may or may not be related to the transmission or VSS sensor problem.
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