I have had the same problem since I've had the car. The speedo and gas gauge don't work at all now, though.
I heard that both the gauge for the speedometer and the gas gauge are grounded in the right-hand kick panel. I'd go mess with it now, but it's freezing (under 28F here), and seeing as I'm a Floridian, I don't even think about going outside in this weather.
I'd bet money that it's in that ground connection/wire somewhere. If there is body paint underneath that ground connector you won't get a good ground. Did you check all connections in that ground wire?
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I heard that both the gauge for the speedometer and the gas gauge are grounded in the right-hand kick panel. I'd go mess with it now, but it's freezing (under 28F here), and seeing as I'm a Floridian, I don't even think about going outside in this weather.