I am trying to run video from a computer video card to an older TV
with a coax cable input only. I am using a composite video cable to
an RF modulator box, then to the TV with the coax. The picture has
lines and is jumpy like the frequency is slightly off. There isn't an
obvious fine tuning adjustment on the tv. I was wondering if its
possible to adjust it, or if theres anything else that could be
causing the problem. Its a zenith, about 10 years old. It works fine
for normal viewing.

any help is appreciated. thanks

Aron