Check the SNR level. Turn to one of the channels having problem
menu>settings>systemInfo then press the blue info button on remote and the diagnostic will load. left side of main screen should say good(fair, poor or na is a signal problem) . short term should be 0.
This does not work on DCT700 boxes.
You may need to change the main splitter if all of your boxes have the same problem. Also try rebooting ONT if you havent already. Open coax line can cause issues as well.