A regular bounce (about 2-3 per second) can be caused by a couple things. Most common would be a drive cable that is binding. The solution to that is to pull out the cable to clean and re-grease it for the full length. The other cause would be if the aluminum disc inside the speedo is bent, so it rubs against the magnet. The latter cause would only be possible if the speedo has been taken apart previously and not put back together properly. So money is on the speedo cable.
A rapid, but small, consistent fluctuation is normally from the odometer gears binding and in need of cleaning and lubrication. The gage has to come apart to do that too. If left too long, binding inside the gage will eventually unravel the drive cable.
A gentle bounce as you hit bumps and such is normal operation of a mechanical gage and cannot be changed. You can even make the tach needle bounce by Revington the motor quickly up and down...just the nature of the design. I once had a Vette that was so fast I could bounce the the speedo needle by accelerating so fast (0-60 in 2.3 sec).