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Speedometer puzzler

The other day a friend was driving my 99 Mercury Sable Le about 30 mph when the speedometer suddenly died. Why did the speedometer suddenly die? The car continued to run but a bit sluggishly. He then stopped and turned off the ignition. He had been driving the car for only 5 minutes and the heat gauge was at normal midpoint where it remained for almost an hour before declining. Why did the heat gauge take so long to decline? At no point did the engine repair light appear. After several hours he resumed driving and all seemed well. And there’s a precedent to this Puzzler. A month ago he as driving the car 55 mph when the speedometer fluctuated wildly from 0 to 50 or 60. He stopped and after 10 minutes resumed driving without a speedometer problem until yesterday. Again the engine repair light did not appear. What’s going on with the speedometer? Where are those Magliozzi brothers when you need them? This is a Puzzler. Please help me solve it.