Sounds a lot like a hydraulic thing to me. Are you sure the slave is moving far enough. I remember the first time I bled the system I was kind of surprised at the stroke distance of the piston. You obviously know what your doing if you can rebuild the rest of the drivetrain, but just to say it, the piston is supposed to move almost the same length of the slave cylinder housing/body.
Good luck I know you hate to pull that damn thing again......especially if you really don't need to.
