Welp, the Roto Rooter guy finally showed. It was the loose screw that was causing the problem. But I didn't have a wrench or pliers strong enough to fix it.
So he tightened it up....real good! Now my toitey's fine.
Meanwhile, the toilet wasn't the only thing that got screwed! The plumber was here for all of ten minutes, and tightened one stupid screw....and charged me $145 bucks for it!
Man, I'm in the wrong business...I should have become a plumber. Maybe I could at least marry one....