Loss of power

I’ve been working on a 1979 Datsun 620 pick up (manual). Having a strange problem. When I accelerate hard over 40 mph, I’m experience a slip in power. Feels like the transmission is spinning without catching but there is no weird crunching of gears in or out. It catches and slips spastically in second and half second increments. I down shift and get the same results until i slow down. This is particularly marked on uphills. New fuel pump. Decent carburetor though not rebuilt yet. Help? Is my tranny busted? Or is there another possible cause?

Clutch is first thing possible. Sounds worn out.

Listen to the webcast of this week’s Car Talk show for instructions on how to test to see if your clutch is slipping.

You are sure that the engine is disconnecting from the drive train, and the engine revs up as the vehicle slows down? I note that you mention replacing the fuel pump which casts some doubt on the actual symptom. If the engine is misfiring or cutting out under hard acceleration, I would check the fuel filter, then the distributor cap, then clean all the electrical connections on the primary side of the ignition.