Hi Bryan.
I see. Doing some tests I got it.
Animating amplitude with step in (no interpolation) reveals the problem: It works but, the moment the amplitude changes, the object jumps instantly to the place it should be if the new amplitude were active since the beginning. If you interpolate amplitude changes, the transition frames are a kind of weird high freq wave, and stabilizes when the amplitude stays constant.
I'm now trying with a different approach: Simulating a real ping-pong with a constant speed particle.
I shoot one particle with a initial velocity, and set two bouncing boxes. The particle kepps bouncing at perfectly constant speed and I can animate the separation of the boxes to change amplitude. It works perfect if the bouncing boxes move gently. If they're moving fast at the contact moment they sometimes miss the ball. And you might need to go to frame 0 and restart the particle system after changing the amplituce animation, but it's only one particle so everything runs very fast. Usable.
Great!, but now, how can I send the x coordinate of my particle to another object? I'm afraid this workaround gets me in the gates of pCustom, something I'm really scared of