OK - one more update, anyone got any suggestions?

Retimed the car and found that both leading and trailing were set to 30 - so I reset leading to 24 and trailing to 16 which an earlier post I found said was a good place to start. Car is still breaking up like crazy at about 5500. Timing light showed good strong flash on both coil wires and all four plug leads. I'm completely stumped. It must be a carb problem, or a very mysterious ignition miss that only surfaces at high RPM's. I've got a race this week-end and am not looking forward to being a moving chicane again. Has anyone ever experienced something similar to this?