More than likely it is just one item causing the interference, but I can not rule anything out. I have seen some weird things cause inference.
Power will need to be removed from the suspect interfering item.
My microwave oven interferes with my D* receiver. Other devices that can cause inference with electronics are induction motors. The first thing that came to mind is the fan in a forced air heating system or a refrigerator. Light dimmers are also a common cause of interference.
Another thing that I experienced was electronic devices interfering with each other by being to close to one another. Once a had a friend that had a stereo receiver mounted underneath a television. The radiation from the television prevented the stereo receiver from picking up one, and only one, of the local radio stations.
Weird things happen!
Could it be frequency related as to why it's happening on only 2 HD Channels, and not on the majority of them?
When testing items in the house the other day, I only turned them off, I didn't unplug them, so perhaps I need to start from scratch on eliminating electric items?
Another strange thing is that so far as I can tell, this problem doesn't start happening until later in the afternoon, but does happen all the way up past 3:00am. I can't think of any items that do this to cause interference.
But then your statement, "weird things happen" come to mind.
Another question I would have is if there was anything I could do to shield this interference?
I already have 2 surge protectors/power cleaners for my a/v setup. I plugged the receiver into the wall outlet to rule out these protectors/cleaners from causing the problem. Below is what is stated about these protectors/cleaners:
Protects sensitive electronic equipment from power disturbances.
Removes electromagnetic and radio frequency interference from household appliances, fluorescent lights, and digital components.
3850J of surge protection, 132,000 Amp Maximum Spike Current
Again though, "weird things happen."