'99 Taurus DOHC SE OverDrive Issues?

I've got a '99 SE with 70,000 miles on it and sometimes when it goes into overdrive on the highway the odometer starts going back and forth from about 40mph to 75mph then will end up going completely off. The tach and all the other gauges still work, it's only the speedo. The OD off light will start to flash and the check engine light is coming on. Usually it drives fine when this happens but this weekend it started bucking and hesitating, enough to make me pull off the interstate and look under the hood. Once I looked, got back on the interstate in drive then shifted to overdrive once it got up to speed, it ran fine and everything worked. It has happened a few times over the past 3 years usually only on long road trips and at first would only happen with cruise on and this is the 1st time the check engine light came on. I haven't had it tested yet to see what codes are coming up.

I've recently changed the plugs, cleaned the injectors, not because of this issue just because they needed it. It only happens at highway speeds and is fine the rest of the time. Do you think it's the CPU or Maybe the Overdrive itself? I know some about cars but nothing at all really with transmission/computer issues..

Any help is appreciated, Thanks.

I would get the codes read. Sounds more like a sensor issue than a major mechanical problem. post the code whan you have them. Autozone reads them for free.

That's what I'm planning on, I think your right that it's just a sensor. I'm just a little worried that actual overdrive might be going. Lets hope for the best. I will post the error code as soon as I can get it read.

bad Vehicle Speed Sensor

Thanks for your input guys, I'm sure your right that it's just a bad speed censor, It's working just fine in the city and the check engine light went off. i'm sure i'll have another question soon, lots of little things are starting to go wrong with it.