Could be a few things.
1) Slipper is loose and slipping too much, especially once warmed up
2) Clutch shoes are worn and slipping too much, especially once warmed up
3) Flywheel nut is loose
4) OWB inside the trans is dirty/worn out
Take the body off and when it's slipping, look to see what spins vs what doesn't.
If the spur doesn't spin and the CB stands stationary with it, look at the flywheel. If the flywheel is spinning, it's your clutch. If the flywheel is not spinning, the flywheel nut is loose.
If you see your spur spinning and not the nut/shaft that holds the spur, definitely the slipper being too loose.
If your slipper shaft/nut is spinning and it obviously isn't moving forward, look at all the axles (center front/rear, outer front/rear). If none of those are spinning and your spur is, something in the trans has given up on you.