Well, I don't what it is if it isn't atmospheric.
I pulled the antenna down and checked everything and reseated the coax connectors. Made a couple of changes including adding another 5' of mast. Antenna is now about 50' AGL or somewhere around 400' AMSL.
I can get stations from farther and with a weaker signal than my target station, but they're UHF and the target is VHF. This all started about a week or less ago. Prior to that, I could get the target station fine most of the day. It would flake out at times or in heavy weather.
I guess the mast mounted amp could be going bad.