Might be an issue with the cable coming from the cab in the street to your house. It has been known for the incoming coax (from the cab on the street to a customers address) to deteriorate with age. If some channels are coming through ok without any issues and your signal level is ok then that could explain it. The cable may no longer be stable when used at higher frequencies. The way to tell if this is the case is to tune into a channel that is in the higher frequency range. If the picture is stable then its most likely a signal quality issue that can be cured with use of a signal booster. If the picture is pixelating or jumpy. This could point towards the cable being no good and a re-pull from the cabinet may be required. If you have a neighbour with a VM connection. Try the box at theirs first.