Do you have a VCR connected to your TV? If so, try these steps:

1. Shut off the VCR. If the problem clears up, the VCR is not properly "fine-tuned." Turn VCR on and troubleshoot the VCR's problem. 
2. The VCR may not be "fine-tuned" to channel 3. Locate and adjust the fine-tuning mechanism until the picture comes in clearly. 
3. Try making sure that the <3/4> switch is in the correct position. Locate the switch on the back of the VCR and verify it is on <3>.