Hi Jim,
Usually, this is combination of keyframe interval; being too low, and/or bit rate being too low.
Try shortening the keyframe interval first. This is a point in the video where the compression provides a full frame, not just changes from the previous frames. If the interval is too long, you will start seeing artifacts on fade ins, and crossfades, where every consecutive frame is different.
Vince Becquiot
Kaptis Studios
San Francisco – Bay Area