Transmission or Clutch Problem???

[Cowboy13]

Driving down the road in my 1970 K20 Chev and all of the sudden it feels like it pops out of gear. Truck lurches and loses acceleration. I can still freely shift it using the stick but the truck has no forward momentum, just revvs up when i step on the gas. Pull over to the side of the road, shut the truck off and start it back up again and it works and runs fine. Any ideas whats going on? Thanks for any help.