There's nothing that could be causing your multipath interference, eh? Nothing that could possibly be a reflector?
What about those 3 antennas below in your setup? Their reflecting elements are indiscriminate.
(For instance... the chandelier in my living room reflects RF 22 so well that I keep my indoor UHF antenna pointed towards it to catch a problem channel.)
Take that specific antenna off the boom and try it mounted somewhere else, I bet your problems will clear up.