No-load Misfire Puzzle

I'm running a fairly stock 383 in my 70 Swinger. The carburetor is an Edlebrock 600 cfm, on a single plane Wieand manifold. I'm using the stock Mopar electronic ignition. When running at speed, under light load conditions, a regular misfire, or hesitation occurs. The engine performs fine under load and hard acceleration. My take: It's not ignition, but a fueling problem. Anyone????