This is REALLY interesting:
Just had cable TV installed. From time to time, I'm getting terrible snow on all channels. While my son was in front of the TV yesterday, I walked into the bathroom and flipped on the light, and then turned it off a minute later. At that moment, the TV picture cleared up. About the light switch: It's mounted in the same box as a GFI-protected outlet. The GFI kills not just the outlet, but also the light switch. The GFI has shut off the power twice at other times, with no apparent reason, since the outlets were not in use, just the lights.
I know it's time to open the box and check for proper installation & grounding, but it's the TV interference that's interesting. Assuming yesterday's episode is repeatable, and not just a coincidence, how would a GFI cause interference? Other info: The TV cable enters the house right by the AC service box and crosses over the line to the bathroom at a right angle. And, I have BX cable.