TV interference can be very difficult to diagnose and it could be coming from many sources.

If I had to make a wild guess, based on your simple description, I might suggest your problem could be caused by "power line" interference. It typically shows up as random/intermitant lines (dots and dashes) running through the picture and (if it is bad) "static" also coming through the TV audio (in sync with the video 'noise'). Usually this type of electrical interference is worse on lower channels (2-6) and starts to fad out (decrease) for the higher channels.

One way to help verify this is to listen to an AM radio tuned to a not-too strong station and see if you can hear the static in the radio when you see the problem on your TV.

If you can coordinate the problem on the radio TV it is a good indicator the problem may be coming from a faulty 'power line', probabaly somewher close to your neighborhood. Powerline interference bad enough to disrupt TV and radio signals is usually caused by loose or cracked insulators, loose connecting hardware on te powerline, bad lighening arrestors or silly things like a piece of chain or wire thrown over a high voltage line. Often times powerline interference is worse on a dry, windy day. (Interestingly/surprisingly when it rains this type of interference often STOPS becasue the electrical arcing is reduced due to the slightly improved conductivity of the water which reduces the electical arcing!)

If you can confirm the interference may be coming from the powerline you may be able to (hopefully) get some help from your local power utility.

Good luck.