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Dear Steve,
The first best answer would come from someone who has actually fixed this in their own vehicle. My reply includes no such personal experience.
However, there is a component on the circuit board of the speedo whose job it is to isolate or decouple the spikes and noise that normally accompany the power input to the speedo unit from the more sensitive circuitry used to count the pulses from the differential pickup. It is a large electrolytic capacitor.
The dashboard environment is unkind to electrolytic capacitors over time (heat) and the result is they dry out after umpteen years and no longer do the job of absorbing the little glitches. A good example is in the clock, where two such capacitors dry up, causing the clock to quit when it turns cold.
A good approach to your '89 would be to find another speedo and pop it into your cluster. I tend to rule out the lousy connector and flex wiring at the differential, as you say the needle pops up just by starting the car. And the traffic jam inching syndrome might be explained by the surge from the alternator when the brake lights are switched.
If the swap of the gauge is not convenient, there is a blurb in the 7/9 FAQs on replacing the power supply decoupling capacitor, though I think it addresses another symptom, perhaps in a Y-brand rather than a V-brand cluster.
Also it could be as simple as a bit of corrosion on the three-pin card edge connector used to bring power, ground and MPH sensor signal to the speedo. They tend to get loose once the anti-tamper retainer disappears.
Best of luck,
--
Art Benstein near Baltimore
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