zooming animation is not smooth in Premiere CS6
Turns out that one should use the Motion effect and not the Transform effect for zooming in Premiere, even though they have exactly the same Position and Scale controls!

Here is the result with the Motion effect

p.s. The Anti-Flicker filter I discovered under the Motion effect was left to the default of 0, so that was not needed.

p.p.s. I also found this discussion of a similar issue