mike wrote:

Vertical lines scrolling sideways is one of the symptoms of co-channel
interference.
At least two possibilities
1) loose connection lets RF from local broadcast get in with the cable.
2) overload in front end, try an attenuator on the RF input.
Oh, and bad cable signal, but you've already complained to the cable
company yes???
mike

The cable company is saying the signal is fine, and the problem just
started even though nothing has changed for years.

It doesn't happen in every channel, so how do I prevent the local
broadcast from getting in?

What is "an attenuator on the RF input" ?

Thanks