If you have a source with separate audio and video (not RF) output, you can temporarily connect that video source to the video input of your TV. Typically, this is a spot directly after the video detector, where it sends the video signal to the grid of the video amplifier tube. This diagram is for a different TV, but it should give you the general idea:
http://antiqueradio.org/art/A-V_Adap...oInjection.jpg
If the picture looks dramatically better when you inject the video, that suggests that you have a problem somewhere upstream of the injection point, in the tuner/IF/detector stages.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html