I haven't followed this thread very closely, but the statement that a
preamp is being used when stations are 15 miles away got my attention.

I'm roughly the same distance from TV broadcast towers here, and I have
to use an attenuator to prevent overloading my TV when using a small
antenna in the attic. Without the attenuator, the TV goes blank on a
couple of channels. I'd be very surprised if your preamp and/or TV
aren't being overloaded, and that might be the cause of problems you're
having.

Roy Lewallen, W7EL