gymsurely wrote:
It looks like the problem is IC501 (TDA8172). Does this make sense? The
picture returns to normal if I tap the part. I'm not an electronics
person but I'm tempted to buy a part and replace it.

It must be some part nearby, maybe even mounted on the same aluminum
plate, but definitely not this IC. Your error is in the E/W circuit,
this IC handles vertical deflection. Most of the time the E/W regulator
transistor or some related coils will have bad solder connections.

--
Met vriendelijke groet,

Maarten Bakker.