this has to do with your OTA local signal strength. While you are in channel ABC or CBS, go to the settings menu and look what the signal strength for you over the air antenna on this channel. Do it for NBC as well so that you can compare the signal strength of the three OTA channels. You might have to re-direct your OTA antenna.
CBS & ABC are definate problem. How is your distance from the transmitter (ABC & CBS)? Is it more 40 miles? If it's not you might have to re-point your antenna to a location where you can get a decent strength for ABC & CBS.