I would guess it's possibly the clutch fork? This goes from the slave, to under the pivot ball in the clutch housing, to the fork end at the clutch. It's possible the pivot ball is broken or worn.
The clutch fork can be removed/installed reasonably easily in the car...can be a little tricky to get the fork back in the right place and the end of the arm back under the ball of the slave cylinder, but it's not rocket science. I would suggest removing it and inspecting: make sure the ball is intact and not grooved or damaged. If everything looks good, apply some grease to the fork, the pivot point, and the end of the slave arm.