I had a Ford 302 V8 sometime ago and it had a similar problem at thigh rpms. In my case, I had to re-ground the distributor "shaft" and its gear. It solved the problem. Not sure if it would help, but you can see it just removing your distributor and moving its shaft. If it moves in an anormal way, it might have a problem. Another thing you can do is to check your carb base to intake gasket to be sure of its sealing to try to make more vacuum. Just trying to help... Tks & good luck!
