I had an old Ford P/u that did that....Turned out to be a cracked rubber fuel line from the tank to the frame rail. When the frame twisted the crack would open and break the suction from the block mounted fuel pump. If you didn't hurry up around the corner it would begin to stumble lean.
Since a carb swap didn't help i would look for a fuel source problem. Of course a loose wire in the ignition could be doing the same thing.
I hope this helps you at least with ideas.