Not sure, but my 1992 940Turbo speedo has been doing similar things for a couple years now. These things are electronic as opposed to mechanical. Last year, I took the instrument cluster out of the dashboard and examined it thoroughly with a jeweler's loupe to see if I could find anything obvious. I found some copper corrosion around a couple of integrated circuit chips, so I cleaned it all off and reinstalled the cluster. It worked fine for about a month, then began acting up again.

I bought a speedo on ebay from a wrecked 940 and am about to go change it out right now. I'll let you know if that does it for mine or not.