This may or may not help, because you have already tried another set of carbs. I had the same problem and chased it for a whole season. After replacing just about everything imaginable on the car I found it at the end of the season. It turned out to be one of 2 things. Because of time restraints at a race I changed the vacume pod diaphram and spring at the same time in both carbs. I made a pass with the car and that was the last time I had the nightmare stumble. I'm leaving off a brake so there is a difference there. Good luck!