I've noticed this on many HDTVs, and I *think* I've noticed it on the Panasonics I have as well. This is just a wild, technically uninformed guess but I wonder if it has anything to do with the scrolling getting out of sync with the TV's refresh rate for a second, and then catching up again. Because it seems to happen only a minority of the time.
1) Is it cable TV signal or Over the Air Antenna?
2) Did you try turning off all the Noise Reduction and Edge Enhancement in the picture menu?
OTA signals are notoriously much, much better than the overdistributed and fractionated cable signals.