I had a somewhat similiar problem..... it was most noticeable
when starting from dead stop in first gear.  Turned out there
was a slight oil leak from the crank case into the clutch area,
and the oil fouled my new clutch disk and started eating
little pits into the disk.   Originally I had a tranny shop tell me
the tranny was just aging and I should drive until it died.  By
25k the shudder was intolerable.  When I took the tranny
out and saw the pits, I took the disk into a different shop
and they explained the oil thing.  At any rate, I replaced
the needle bearing and the crank seal, and put in a new
clutch disk.  25k since, no problems.  Rats.
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Steve E
84 Vanagon
Lansing Mich