Taurus - Speedometer/trans

My 94 Taurus just started doing this: (This is my 4th Taurus and my first problem)
Speedometer doesn't work for about 7 or 8 minutes into the ride to work.
Then it starts, but it oscillates a bit when it does work.
If you use the brakes when the car is first going along you *might* get what you could call brakes grabbing or transmission grabbing as it downshifts - I can't tell which it feels like. But with the speedometer thing it is making me wonder about some transmission problem. I've tried quite diligently to produce the problem without using the brakes, but can't.
If I go to work and make a conscience effort to not brake (within reason) I have no symptoms other then the speedometer trouble.
Any thoughts as how to troubleshoot this or if someone has had the same symptoms/
Thanks, Norm
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The speedo is old news, they all do it from what I've read. My has done it for five years, I've learned to drive by the tach.
Cecil

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sounds like a speed sensor concern, has it been tested for codes?

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