No more calls please; we have a winner. Someone in
sci.engr.television.broadcast nailed it on the first try. There's a 99FM
xmitter a couple miles to the SE of me, lower in elevation, right in the
path of all the
other stations - & right smack in the middle of the FM band. Pointing the
yagi in the exact opposite direction yields the best results for all other
stations.

I'm still looking for some vertical omni antenna designs.

-Bob