Mine also bounces somewhat. I've been advised by my builder to check the speedometer knurled connecter behind the speedo - the connector takes a fairly aggressive turn/angle and loosening the connector very slightly seemed to unbind the cable - it still bounces - just not as much. Try loosening first and test driving - then maybe tightening. Let us know what you find - you're not alone.
Anyway, over 100 MPH dont' be looking at that gauge - looking ahead might be safer
Be safe -- Mike