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I have very similar problems, but my situation is reversed. I have a 90 cluster in an 88 car. I was able to get it to work by running an additional wire. From my understanding of the way they work, it's not so straightforward to put an older cluster in a newer car.
Another issue is that the number of splines on the tone ring changed between the two years, so the speedo from the other year won't read correctly. How do I know all this? I put a 90 rear end in my 88 wagon, to gain the benefits of a 3.31 ratio, (29-31 MPG !!) when I changed the car from automatic to overdrive. I found that the speedo and odo then ran at half speed. No problem, I thought, I'll use the one that matches the rear end. It was then that I found the problem with the incompatible wiring. After figuring that out, I was disappointed to find that the speedo still reads incorrectly, with an error that increases with speed---it's about right up to 25 or so, then lags behind the true speed, 50 MPH actual reads only 40, 65 actual = about 48. The weird thing is that the odometer is within 2% of correct at all speeds.
I also have the same intermittent problem as yours. At first I thought it was the wire to the speed sensor, which was in fact bad, but not the cause of it. If you have ABS, the ABS light will come on if this wire has a fault, regardless of what the speedometer is doing. With the wire replaced, the speedo and odo are still intermittent. I gradually became aware the problem is related to ambient temperature. If it's cold out, it won't work. After the heater has been on for 10-15 minutes, it will suddenly spring to life. I suspect the oil in the gauge itself has turned to thick goo. One of these days I'll pul the cluster out, take it apart, and try to clean and reoil it.
If the speedometer ONLY is intermittent, and the odometer runs normally, the problem is electric or mechanical, within the instrument, and replacement is the best option. The three wire clusters were used up to and including 88 models. 89-90 have the four wire type. 91 and up have the new style dash---completely different. There may have been other changes over the years too.
Hope this helps. If anyone has info about the difference between the various speed sensors for the rear axle (there are several types) I would be interested to know if they affect the speedometer reading---could this be why mine doesn't read correctly (when it feels like working at all)
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