Tried applying heat from a hairdryer to the line o/p stage and also replaced the FKP original snubber cap - no joy. I vaguely recall having this exact problem with a 14" Sanyo set of similar (early 80s ) vintage about 20 years ago, and am racking my brains as to what the cause was.
Here is a brief video showing the symptoms. As you can see there are horizontal bands of defocused interference.
I attach diagram of the line stage in case it jogs anyone's memory!