Your clutch release bearing carrier is hanging up on the front cover of the gearbox. As the carrier and cover warm up during use, the carrier-to-cover clearance is reduced, resulting in the symptoms you describe. Only solution is to R&R the gearbox and hone the cover and grind an angled relief on the cover's end. Eventually the carrier might wear the cover enough to reduce the "sticky clutch" syndrome to tolerable levels.
See the Buckeye Triumphs website for the details.
Good luck.
Rick