1986 Ford F-150 slipping clutch

The engine was rebuilt about 30,000 mi. Ago, clutch disc, pressure plate and throw-out bearing were replaced at that time. Truck is not used often, maybe once a week. Problem became apparent suddenly when starting out. As the clutch is engaged, pedal is released fully while the clutch itself can be felt slipping. As the truck begins to move the slipping seems to ease and feels like it is starting to engage normally. If the engine is reved the clutch slips but as the revs are reduced you can feel it begin to grab. It seems that the more you drive it the better the clutch seems to work. There are no problems as far as shifting into gear or changing gears once the truck is in motion. I have checked the master, slave and hydraulic line for leaks (there weren't any), also topped off the fluid.