My son's 95 has the same problem. From searching for the solution I found an information that this might be a bad circuit board inside the speedometer. Somebody was able to buy it on Ebay and fixed the problem. I could not find any boards anywhere but I never looked for them aggressively enough.
I did not want to buy another speedometer since I was not 100% sure that that was the problem. If you are lucky enough to fix it, let us know.
Sam