If the degauss is working ok, it'll be the shadowmask. Enjoy it as it
is, or if you cant do that, try giving it a good hard whack in a few
directions, which will either fix it or kill it for good.
Both myself and brother have this TV. We had the same problem. Your
set needs to be degaussed. That's it, PERIOD. It is a magnetic loop,
plugged into AC, waved in a circular motion around the front of the
tube on your TV. That's it. Rent or borrow one. Dirt cheap to fix.