I have got an all stock 1985 GS with 60,000 original miles. It sat for a long time but is now driven almost daily. The carb has been rebuilt, fuel pump replaced, tank reworked due to rust, and plugs, wires and distributor cap replaced.
The car idles well and accelerates well.
When in a no load condition, ie going on flat ground, or down hill, the engine seems to stumble and the car bucks slightly. It is more prevalent between 2-3K rpm and does it in all gears. It is more noticeable at slower speeds.
While the car is drivable, it is annoying.
With the car in neutral and the engine running between 2-4k, the engine will run fine, suddenly stumble or miss, and then recover. I can feel it while sitting in the car, and the shifter will jump slightly. There is no RPM drop when this happens.
Any ideas?

 

No ideas?

 

Sounds reminiscent of an ignition-based misfire. First step would be to look toward your electrical connections, and distributor, including the advance mechanisms. 2-4k is within the region where advance is taking place.