Intervals were not designed to be interactive. A pos interval sets an object’s position each frame based on a pre-calculated interpolation, rather than incrementing the position based on its current position.
This is great for turn-based combat animations distributed over a network (like in Toontown) or for pre-scripted sequences.
It is not good, however, for anything resembling interactive physics. You should write a task if you want increment a position every frame instead of setting it.
You can get some leeway by setting [color=darkred]bakeInStart to false and [color=darkred]fluid to true, but this is still not recommended for interactive physics.
See the manual page for exactly what these parameters do.