Platform motion occurs when there is another signal on the same licensed frequency. The condition only happens when there is a slight difference in actual operating frequency. I have never heard platform motion either.When was this? I was working at WJDX-AM when they were AM stereo in the late 80's. I never noticed the platform motion on my Sparkomatic car radio or my Sony portables. I do know platform motion was an issue early on, as CW stated.