Check the obvious first.
What are you using for your input signal?
Are the lines on all the channels?
Even if you have cable, there might be a problem with the signal to the TV.
Before you do anything to the TV itself, check that that input cable, wherever it's coming
from is not causing the problem. The second important thing to check, is if you have
some new or relocated electronic device near (or close) to the input cable.
You may have a circuit problem, but move the cable around some, to see if you inducing
a stray signal onto the input. An actual antenna will be even more prone to this type
of a problem.