Okay, at that speed I can see it happening.
The first trace shows the video when the interference is absent:
The second shows the culprit when interference is at its peak:
The circled pip starts at the bottom of the trace and glides straight up, pauses at the top, then moves down again. Inteference is worst when it's at the top, absent when it's at the bottom.
The interference exactly matches the movement of this thing . . . whatever it is.
Edit: I added a link to a brief video that shows the demon pip in motion. Watch the left edge of the trace, and note the audio buzzing that accompanies the interference:
http://antiqueradio.org/art/temp/Ver...terference.mpg
Phil