Is your internet connection through Comcast as well - essentially using the same "line" from Comcast for both cable TV and cable Internet? If so, it could easily be a problem with the Comcast line, cables, or connectors. Have Comcast check them out.
You could also disconnect your computer from the network entirely (even the local network and even if you need to turn off the router or dsable the connections to do so) and see if the line still occurs on the computer using just Word for example. If not, then it could have something to do with the network connection. If so, then it's either outside interference of some sort or something with your computer (and, hard as this is to believe) also with your TV at the same time.
Have you installed anything new recently in the home - like a new microwave oven or started using some sort of space heater or a new stereo system or radio signal booster or HAM radio or wireless telephones or new cell phones or similar devices or anything like that? If so, turn it/them off and see if the problem goes away.
Are you using a wireless router? If so, it could be broadcasting differently and causing the interference with both devices. Try turning it off and see if the interference goes away. If so, it's probably a hardware problem with the router. If not, it probably isn't the cause and you can power it back up.
Check with your neighbors to see if 1) they are experiencing the same problem and 2) if they've installed any new transmitters or electronic equipment recently (I've known people who bought high powered servers meant for businesses which caused this type of problem - I bought such a server through my company but it caused no troubles but they do come with an FCC warning that they could). This may help identify the scope of the problem and perhaps even the cause.
Do you know anyone with a laptop with a wireless connection? If so, invite them over and see if they have the same line on their laptop when used in your house. If so, you'll know something there is causing it. If not, then I honestly don't have a clue what it might be. Perhaps an electrician can test your home for some type of electronic or magnetic interference (I'm not sure what other specilist to call who would be better equipped).
Though a hassle, you could always buy a new TV and check for the line. If it's there, then there's something in the house or area causing the problem. If it's gone, then I'm again at a loss to explain it (but I guess you could either replace the TV or return the new TV for a refund having completed the test). I suppose you could do the same with your monitor (or instead with your monitor) but the TV seems a more definitive test.
If you want to test the computer more, then post back and we'll try some other things that are more related to the general function of the computer than to the graphics which we've pretty much addressed as much as we can already.
I hope this helps or at least gives you some ideas to try.
Good luck!
