Had this problem myself some years ago. The problem was not that the clutch slipped once it was properly engaged but that the clutch would not engage at all.
I pulled the gearbox
down and found that there are two thrust washers activated by bellvue springs - conical washer things, on the clutch shaft. The forward clutch thrust washer had a slight circumferential groove worn in it and I theorized that the spring and thrust washer arrangement was there in order to prevent the clutch cone from sagging and miss-aligning.
These transmissions work by having either the cone or clutch engage by traveling both axially and tangentially along the shaft - they go back and forth along a helical spline and consequently the more torque applied the tighter the cone and cup engage (This is the reason that you put the gearbox
in reverse rather than forward if you want to stop the prop rotating when sailing) If one or the other of cone or cup sags on the shaft they just touch at the top and slip without fully engaging.
All I did was turn the thrust washer around so that it presented an unworn surface to the bellvue spring and I had no further problems with it. If you want new parts
I would replace both thrust washers and bellvue springs however if you do this you should go through the full spacer adjustment procedure on reassembly.