What happens with the raster when there's NO input connected at all? (this could be a way to trace the interfering source).
If you have any GFI (Ground Fault Interrupt) distribution panel breakers or GFI outlets in the house, they too might be a problematic source of your RFI (radio frequency interference). Flourescent or halogen plasma lighting can even be an issue, but apparently not in your case
At one point, the only breaker that was still on was the one for the TV.
Regarding possible differences of ground potential on your incoming RF coax: you can try grounding the shield connector by stripping a few inches of small conductor (#22 awg or so) insulated wire & wrapping that around the F-connector shield, fastening it with a cable tie or tape wrapped around. Then connect the other bared end to an AC outlet cover mounting screw, which typically ties electrically to the AC outlet's ground. Or you can go right inside the outlet box & pick up the AC ground connection directly.
I've had to use this shield grounding technique for video hum bar problems; others have used it successfully too. Your situation seems a little different, but this is simple & easy to try.
You can also try completely isolating the coax by connecting two antenna matching baluns directly back to back (available at Radio Shack). The incoming coax connects into balun #1, then the 300 ohm twinleads tie directly to each of balun #2 twinleads. The 75 ohm isolated coaxial output is then taken out of balun #2. This is inexpensive & introduces some small signal losses, but if you have signal to spare then it's not a problem. There's a $100 coaxial isolation box available (from MIT?) that may or may not(?) introduce smaller RF losses. Sorry I don't remember those specs or the brand for sure.
There could even be a source of external RFI getting into your equipment (nearby radio station transmitting tower or cell phone tower). Or a neighbor's CB radio or Amateur Radio transmissions. That might need to be shielded or trapped out. You can try a product from Audioquest called RF Stoppers. Basically they are split ferrite rings that come apart & snap together around your cabling (RF &/or power cables, etc). There are two levels: the Juniors & the Regulars, which are twice as thick & yield more RF filtering attenuation. Various diameters (I think?) are available. I saw a similar product at Radio Shack years ago; might still have them too.
AC line conditioning is also worth trying of course. If the power cord is a detachable IEC type, some upgrade AC cords even have built-in filtering (such as JPS, MIT, or Custom Power Cord Co.) and many others. These cords have sometimes been reported to significantly improve video quality, even where interference is not an issue. Search the forum archives for specific posts in that regard.