Diagonal Noise lines

I am having a problem with my video in my home theater.. There are diagonal noise lines rolling through the picture in only some of the channels. For troubleshooting, I have viewed other TVs on the house on the same main cable TV feed, and the noise is not there. This leads me to believe the cable signal is OK. For troubleshooting, I have moved one of the good TV sets with a known good video input cable into the room, and the problem still exists. I have also ran changed the connections on the splitter and the problem still exists with this one location. The only thing that I have not done is run an extension cord from one of the good rooms to troubleshoot the power. My power is going through a Newcastle power conditioner, but I have tried other power conditioners in the same room with no change. All of my video cable have been swapped one by one with the "good" system with no problem. I am on a concrete slab with steel siding. Any ideas??? I am pulling my hair out...
could be a ground loop issue? Is your incoming Cable isolated by a transformer? Many times attaching a house ground to the coax's sleeve will clean up noise issues, but the usual complaint symptom there is hum in the audio. However I'd first ensure that you are indeed isolated. Many archived threads on this: please try the search engine or just go into the older threads & look. They have suggestions on particular isolators & their prices posted there.
Thanks for the response Bob.

I found the problems, and here are the solutions (for the record):
- My system had a ground isolator, which was the common denominator not checked previously. Removing the Ground Isolator cleaned up the cable signal, but brought a hum back into my preamp. It appears that the ground isolator turned the subsequent cabling into an antenna -- only the local channels had the noise.
- To remove the hum from the preamp, I found the ground bond on the outlet bank to be faulty. Refurbishing this ground connection lowered the noise floor. Additional troubleshooting found much oxidation on the house main ground on water pipe and also at the ground rod for the cable. I was able to see about 400mV of noise traveling on the ground with an Oscope. Installation of a new water pipe ground connection, grounding rod and clamp for the cable system caused the entire ground system to have no detectable noise with the an Oscope measurement.

After a test of all channels on the HT, the picture on all household TVs are perfect (better than they were before). The audio is also improved due to the lower noise floor, increased dynamics, and more subtle detail which is now present.

I guess the devil is in the detail, and this problem has continued to grow over a period of time, changing with weather conditions and other factors I had not been able to quantify. Now it all makes sense. I had reached my frustration point after 20hrs of weekend troubleshooting.

JK: interesting report! Curious how are you able to measure noise-to-ground with the scope when ground is its normal reference? Perhaps you float the scope & attach the scope ground to a known-clean-contact ground? I'm wondering because I suspect a similar corroded-ground situation at my house where the noise floor changes at random, from dead quiet to a low level buzz & then quiet again. Seems worst during the day when electrical loading is greater.
I'm also transformer isolated from Cable + attached a house ground to the incoming Cable's sleeve. I think I'll try removing Cable altogether just to see if anything changes.