Looks like signal reflection.
You've got two cases here:
i. It's recorded onto the tape, this is almost certain if it was an 'off air' television recording - removing it is a nightmare.
ii. If it's a prerecorded tape it's probably an issue in your signal chain.
It's an issue with the way analogue video works tied in with poor terminations, badly sited antennas or faulty equipment. It causes the sudden and uniform change in luminance (brightness) value to reflect in the cabling of the equipment. Imagine a wave hitting a brick wall, it ricochets backwards and forwards until its energy completely decays.
On off-air recordings it's either caused by poor cabling -or- multipath reception where the recorder is receiving two slightly different signals (for example, the 'main' signal, then another weaker version that's bounced off a nearby building) or a combination of both effects. As these signals are slightly out of time this also shows up on regular, high contrast areas such as text. Removing it is a complete pig and you've got to be seriously dedicated to remove it.
Less likely on pre-recorded cassettes, in which case I'd suggest removing the Panasonic as a first pass diagnosis to see if this effect is stopped or deminished.
It's most noticable on text as it tends by its nature to be high contrast, and it's regularly shaped so it's more noticable.
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