Bouncing Speedometer

I've been struggling with a bouncing speedometer. It's been going on for about 6 months now, but until recently it didn't happen enough to leave it with my mechanic. Now it happens on most trips, sometimes more, sometimes less. But not usually continuously.

The speedometer drops suddenly from driving speed, and then jumps back almost immediately. It can be sporadic, for only a few times in a trip. Or it can happen continuously for longer periods of a time, like a half hour. Worst case, it bounces every 10 seconds, like clockwork. In the last month or so, there has been a corresponding relay-like click in my dash, just to the right of the radio. But when it first started months ago, there was no click. And it doesn't always click now. If it does make a click, the dome light goes out. No click, no dome light goes out. Headlights no affected.

If the speed is right, the transmission does not budge. But if I am at around 30-35 mph, the transmission downshifts to accommodate the speed "change" which revs the engine, but then immediately upshifts when the speedo bounces back.

I know the speedo is electronic, and I know there is a speed sensor is on the rear transaxle. But I don't think that's it. It does not seem to correlate to temperature outside, or whether it is raining or not.

I have noticed that the brakes lock up a little when braking at moderate speeds. A little tire squeal. Fortunately, I know how to get off them since I started driving before there were anti-lock brakes. So, I suspect one or more of the wheel ABS sensors isn't working. But I have not looked into testing those yet.

I don't get any lights on dash, and there's nothing on the computer when I read it. As far as the computer is concerned, everything is normal.

 

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Any luck? I've had a similar thing happening. It goes away when it's really hot outside and gets much worse when its cold.