Alright lad
I had a long think and some searching on the net to help you with your problem, and ive tried not to mention anything that has already been said, so here we go..... Assuming everything looks in place and kosher bar the bit of trouble your having, have you had a look at the Brass Bevel Cone that the Flywheel slots onto? maybe this could be worn and in need of replacment. or the only other thing i could think of is maybe the Con-Rod Bushing that connects to the Crankshaft could be worn.
just a couple of stabs in the dark lad, but i would disassemble the clutchbell,clutch flywheel etc and have a good look around.
Paul