Erattic speedometer

I have a 1994 Z28 Camaro. On occasion, when I start to drive I notice the speedometer needle does not move. If I turn off the engine and restart, the needle works fine. It MAY continue to worke fine for several weeks of only a day or two. Again, if I turn off the engine and restart, the problem is usually resolved, until the next time. My mechanic, who is VERY GOOD, just kept the car for two days, checking everything he could think of, but he could find nothing to fix. The speedometer worked fine the whole time he had the car. Has this happened to anyone else? How was the problem resolved?