If they're like mine, they're analog (station numbers below 100). I have had some digital cable boxes (Scientific Atlanta Explorer 2000 and 2100) that wouldn't convert the analog signal carried by most analog stations to a PCM bitstream for the box's digital output - that would produce what you are experiencing, but I'd expect it to affect a lot of stations (nearly all of the channels below 100). Most newer boxes (including the Pace boxes I've used) will provide an A/D conversion for those stations so you have a PCM digital signal from analog-only stations.

You might trade out your box for a new one to determine if there's a problem there - either an older box that doesn't do A/D or a fault with the box.