now for something completely different...............[wasn't that a monty python line ?]
for a disclaimer, i know nothing about manual transmissions.
with that said, [bear with me now, please] my wife's 1994 chrysler concorde would chatter the converter clutch if speed was held around 2000rpms or so.
i don't know where i read this, or who told me about this, but adding a tube of limited slip differential additive [i used some yukon i had on hand, but more than likely, any brand will do] cured it on the first drive, and it's been fine ever since, about two years ago now.
i have no clue if this is anything to think about, just tossing it out there.
just my personal experience in life and its results.
your mileage will vary.