Do you have any other source that you could connect for a comparison? Digital converter box, VHS tape player, etc.
Is the problem the same if you switch both the modulator and the TV to a different channel?
Interference can come in many forms: computer power supply, light dimmers, fluorescents, etc. I even discovered some new LED floodlights that create interference.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html