If it's just certain channels, it's likely that it's your cable box not receiving a clear signal. You might check to make sure your cable input to the house is properly grounded, and then see if the company can do a remote reset on your box.
Keep me posted.
We've had this 720 plasma for about a month and we notice that when watching certain channels there appears to be purplish vertical rolling lines going up the screen. Any reasons for this and is there a solution?