There's no reason that should happen.
Which background setting are you using for your Motion Title?
For the background setting, I'm using Freeze Frame. I'm "Freeze Framing" a photo.
I figured out what was causing the bounce. The length of the Motion Title was slightly longer than the amount of time the play-controls bar appears (across the bottom of the screen) when viewing in full screen. Thus, every time the play-controls bar disappeared, it made the Motion Title (appear) to bounce. In reality, the Motion Title wasn't bouncing... it was simply the screen bouncing/adjusting to the fact that the play-controls bar disappeared. I wasn't accustomed to this because all of the other media players I use don't behave this way.
Glad you figured it out, Bruce. Please mark this question as answered.