We are having issues with the screen freezing up for a few seconds every so often. It will happen several times a day and sometimes several times in short order causing us to miss some of the show/event we are watching.
This could be caused by a weak signal, which can affect only certain channels. Check the connections to the cable box, especially the incoming coax, and eliminate any unnecessary splitters.
To check signal strength with a Motorola box, tune to the offending channel, turn off the box with the remote, then within two seconds hit "OK/Select." This will bring up the diagnostics screen. Select "Inband Status" (probably page d04) and look for "SNR" for the tuner(s). This value should be in the mid to upper 30s. You may be able to improve the signal strength by adding a signal amplifier to the incoming cable line.
It also may be a weak signal caused by problems outside your house, in which case a call to 1-800-COMCAST to have them send out a tech to check on it may be of benefit.