The problem is definitly the clutch. The small pins that hold the clutch in position haved sheared. Before buying a clutch reair kit(way overpriced) try this.
Remove the old sheared-off pins, and replace with solid pins made from a drill bit of the same size. The clutch is probably fine, it just has to be re-secured with new pins. The factory pins are the weak link in this poor design. I learned this the hard way. After replacing all the pins I never had any more trouble. I now have a ST4000+ MkII, which was re-designed & is great so far.