Originally Posted by
Roasted
... the derailleur (I'll admit, a low end used one) exhibits this problem when it appears to be in fine condition, whereas a newer derailleur solves the issue...
If the problem derailleur wasn't new, there's a big pile of things that could have put it out of whack. "Altus" and "used" are never things I want in an investment. Just because you never crashed it doesn't mean the original user didn't do something to bonk it out of line.