The... parallelogram? I googled it quick but I still found myself a little confused over what that is exactly. Could you go into a little more detail about what the parallelogram is on the derailleur and how it works?

Thank you for your suggestions so far.

EDIT - The only other thing is, I also had an older Alivio rear derailleur I put on the bike, and it too acted up. I have to wonder if that shifter has different issues, because on the Altus, it only slips under load, whether it's me torquing up a hill slowly OR speeding up to go down a hill. As long as I'm applying high torque, the Altus has a chance to skip. The Alivio on the other hand ghost shifts quite a bit, causing similar headaches.

Do you think the Alivio vs Altus problems are unrelated? Like I said, with the new LX derailleur in place, all issues are gone...