When did you start having the problem? Does it happen all the time? Have you checked the gas tank for rust?
I was having a similar problem, and went through the entire ignition. Then I realized that it occurred consistently after hard driving (sharp fast corners), and after refueling. I pulled the tank, and it was filled with rust. I sealed the tank, rebuilt the carbs, and with the help of this board, learned how to tune the webers. My -6 has run smooth ever since. Check out the post by T Bird in this link for weber tuning instructions:
Weber rebuild
Justin