Sorry, guys, I have to disagree with the oil. Oil would tend to make it slip, progressively getting worse; I've been there a few times with an early 850 "wet" crank.
Shuddering most likely caused by glazed flywheel or pressure plate, or even the disc itself. Resurface flyheel, replace pp and clutch disc should cure it.
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Could the clutch plate have become glazed? Does it look shiny on part of the plate's face?
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Or bluish, orangish, brownish in color? Unevenly around the flywheel? That would indicate local hotspots (no, not swinging nightclubs /ubbthreads/images/graemlins/wink.gif )where it's become overheated.