Sounds like you will have to remove the engine and inspect the clutch. AND it sounds like it could be 2 problems.......... Could be the release bearing causing "some" of the problem. GRINDING Could be the shaft that holds the release bearing also causing "some" of the problem. GRINDING Could be a bent clutch disc, warped flywheel or pressure plate or even some problem with the splines on the transmission causing the disc to not move freely on the splines. GRINDING
But they should not cause clutch slippage unless there is not enough free play in the clutch cable. SLIPPING Might be oil on the clutch disc, disc worn down, or the wrong disc with its springs hitting the flywheel. SLIPPING Might be a weak pressure plate. SLIPPING
It does sound like it could be just a clutch job though. ;-) OH and make sure that the engine is bolted to the transmission well too!
8^)
JMHO