Speedometer jumping, and inaccurate

A couple of years ago, at the end of the cruising season, I got on my duster really hard. After I slowed down, I noticed the speedo jumping around. Figured that I broke something inside. Put car away for winter. Over the winter bought new dash insert with speedo and guages. Of course, come the spring, the speedo was now fine. It was very accurate before acting up and now its still as accurate. Go figure.