Doesnt sound like a fuel problem to me. High RPM's require the most fuel and you say once it gets to that range its OK. The problem seems to be in the transition area from low RPM's to high RPM's. If I were working on it and I was certain the simple stuff like plugs, wires, points etc. were OK. I would pull the distributor and clean and lube it, it sounds like the advance is frozen. If it is not advancing under accelleration or load you will have the same stumbling, missing situation at mid range.