Ok, this whole time it was a rusted out connector on the outside diplexer. So, new wiring job to follow very soon. I made some temporary fixes.
You would not believe the increase in quality when I replaced the rusty connector with a fresh one. The signals we're better, and the motor moves so much more efficiently. I can actually hear the motor from inside my room. That's how loud it is.
Plus, I found out I can use a multiswitch in place of a diplexer temporarily til I get a new one. Sort of works, but not as well as a standard diplexer.