Sounds like you have a potential problems in a couple areas.
First I would dissaseemble the clutch. Make sure that the flywheel is snug on the shaft, and that the flyweights are not jamming ior sticking in the flywheel. Make sure that the clutch spring is set at the correct distance, and that you have the correct clutchbell endplay. If that all specs out you may have a probelm with the first gear oneway bearing. It may be slipping. You may one to try cleaning it thorughly and or replacing it. Just afew ideas.