I had this problem on my 2wd, its because there was vibration on the input shaft causing the release bearing to break up, the possible cause was a sheared dowel on the flywheel as i noticed a burn mark on the flywheel in the area of the sheared dowel, i presumed this was the problem as it seemed the most obvious but the problem is that i changed so much during the change including new box, clutch, spigot bearing, clutch arm pivot, machined flywheel with new dowels that i couldn't be that sure what the problem was but the sheared dowel seemed the obvious explanation