About two months ago, the clutch started giving off symptoms. It's been vibrating when changing into first from idle, and a good bit less between first and second gear. Hardly noticeable in the other gears. Upping the revs helps a lot. The vibrations have come and gone several times, usually lasting about two weeks, and then away for another couple of weeks. I bought the car used, so I don't know when the clutch was last changed. It takes about 80% out, but it hasn't been slipping on high load in fifth gear yet. I recently drove up an incline with four large and one small man in it in fifth. I paid attention to the rev-meter and engine sound, but got no slip-indication, and felt no loss of forward momentum.

What might this be?