If it's happening on certain channels, it's likely that it's a cable box connection and not the television. You might want to notify your local cable company and see if they can do a remote reset on your box or apply any applicable firmware updates to the cable box.
Keep me posted!
Model PN41A450--we can see scrolling purplish lines constantly rolling up the screen while watching certain channels. We have comcast for our cable provider. It's not a problem when watching DVDs or playing the PS3. What could be the issue and a possible solution to this. Thanks.