I don't have a clue, but I did have sort of a chatter, and it was only in lower gears. I accidently cured the chatter when I replaced my alternator and put new rubber dampners in the alternator drive housing. Just a shot in the dark. jimsjinx
I have experienced the 3rd-4th gear issue here is how I remedied it.
The type and weight of the engine oil you are using makes a difference for me, I use Cheveron Delo 400 LE SAE 15W-40 (non syn) which helps a lot with the 3rd-4th gear shifting.
Also pre-load the shifter just prior to pulling the clutch and then give a firm follow through on the shift.
Work on getting the engine RPM so that the load is off the drive shaft when you shift. Try this going down a hill and see if this make a difference.