I don’t think the problem is with your TV. I think it’s with your cable box. Most CRT televisions in my experience aren’t digital. If this isn’t a digital TV, then the artifacts you are seeing are coming from upstream of it, since I’m pretty sure they are digital artifacts. Just to be sure, do you have another TV to test with the converter? If the problem persists with another TV that is known not to have the problem when working with a converter box, then you can eliminate your TV as the source of the problem.