4-speed chatter problem

You really should check your belly run out and make sure you have the right bellhousing and bearing retainer match.

Seeing how you went with the bearing, is you crank drilled for a bushing? Or, the better question, it it finished machined for a bushing? The pilot shaft could be hanging up in the crank.

I'm just spitballing, it's something to check. I've also seen this on engines with heavy cams and an improper tune. You start letting off the clutch, the vacuum drops right off and now you're fighting the engine, too.