Hi!

I didn't see two postings...

The strange thing is that, when an external antenna (Outdoors) is
connected, these diagonal lines appear to go away?

It is possible that your TV set generates enough interference internally to
cause it to show up on the screen as you watch it. The interference could be
generated by something else in the same room or on the same electrical
circuit as the TV. Have you tried taking the set to another room (even
outdoors would be OK for a test) or a different electrical circuit?

The interference going away when an external antenna is hooked up suggests
that something nearby the TV is actually the culprit.

William