What is causing the 1979 Datsun 280zx to lose acceleration in 2nd gear and on?

I would look at several things. I would start by checking for vacuum leaks, such as intake gaskets, injector seals, vacuum hoses, etc. I would then check compression across all cylinders to make sure they are within @ 5-8 psi of each other. More than 10 psi difference could cause the engine to lag. The greater the loss of vacuum or compression, the more exaggerated loss of power gets. I would also check the ignition timing. It dont have to be off much for lack of performance. Good luck!