OP -- Indeed, as Sweet says, I had the *exact* same issue. I replaced or rebuilt *everything,* even the wiring going to the distributor from the ignition. Nothing solved the issue of that bucking under load in 4th gear. Turned out to be carb icing. Check your heat riser flapper.
Another easy thing to try -- and this sounds stupid, but it works -- wrap your carburetor in aluminum foil. The idea is trap heat from the manifold to keep the carb warm -- even in warm weather. It that cures the issue, which it did for me, then you can confirm carb icing.
If it's stupid but it works ... it isn't stupid.
