If you notice that the ball sticks or hits a bump, do the following

1) Take the ball out and clean it.
2) With the ball removed look inside. You'll see a wheel and 2 bars perpendicular to each other. More than likely there will be dirt/dust wrapped around the the middle of each bar. Take a pointed knife or tweezer and gently remove the dirt/dust that's attached to the bars. Gently turn the bar to remove the dirt/dust completely. This attached dirt is what causes the bumping/skipping.

Number 2 is the most important step. I bet if you do this to the old mouse, it will work perfectly.