I've also had the exact same problem in the past on a 454 mag. Ended up that I developed a soft spot in the fuel hose feeding that engine. You couldn't detect it by just feeling the hose for soft spots. Ended up finding out by hooking a portable fuel tank to the engine that had the issue, ran it up to WOT and problem was gone. Ran a new fuel line to the engine and problem solved. Still was very difficult to find the soft spot that was collapsing on the old hose after it was replaced.