I also have suffered some problems from my speedometer. Mine wasn't so much "crazy" as it was "sticky". I would take off from a stop and the speedo just wouldn't move. Then, once I hit a bump or tapped the instrument bezel, it would start working. Recently it doesn't seem to be as bad though. Maybe it has something to do with the time I removed the instrument cluster when I completed the oil gauge conversion...??? Hmmm...
Best of luck!