I noticed yesterday that despite having a decent TV connected to a digital cable box, that the image was still ghosted, presumably due to noise. Moving cables around didn't fix the problem, but in the process I noticed composite video output on the cable box. I took a shot that using composite would produce better quality signal and plugged an RCA AV cable in. Viola! Ghosting went away, and the picture is much less blurry. I wouldn't have expected coax to be more susceptible to noise, but perhaps the circuitry modulating the coax signal was picking up noise that the composite video circuitry was not.
Problem solved.