[buzzinhalfdozen]

Ben, hate to hear you're still chasing this problem. After reading all the posts here, looks like every avenue has been explored. The post about the carb. being set up for W.O.T. seems like it could have some merit, that would be easy to check, simply install a "normal" carb and test, sure it's not a race set up but under no real load it should quickly tell you if that's the issue.Broken valve springs could be an issue however I'd expect you would only encounter the misfire at a higher RPM range, unless of course they're "really" bad. When the engine starts to "break up" does it have smoke out the headers?Black or dark grey smoke would indicate 1 of 2 things, insufficient ignition events or being overly fueled.AS I've posted before, borrow or buy an MSD ign. tester ( this is an invaluable tool) put the spark tester on the coil and spin the system up to redline, paying close attention to the "look" of the spark.IF you can trigger the system to redline with no loss of spark you know your box and coil are up to the task.I don't believe I've been to a race in the last 4-5 years that I haven't had my MSD tester on my own or someone elses car. A leak down or at least a compression test would be in order, a cheap valve spring pressure tester would be nice to verify at least that all springs are close.Moroso makes an inexpensive one , not sure how accurate it is but it would show a weak spring I'm sure. Flat lobe on cam should be easy to recognize with valve covers off, spinning engine while observing each rocker 1 at a time. While this problem does seem to be very odd, the only SURE way of resolving it is with proper inspection and testing, of course we can always hope to stumble upon the answer, but that requires much more time and effort. Please consider some of the pinpoint tests provided as I've found them to be invaluable tools in diagnosing issues such as the one you're experiencing. Joe 1 other thing that caught my eye, you posted your batt. neg. cable goes to the frame near the batt. and your engine is grounded near the front on the frame, this is correct? Using the frame as ground is not advisable, Running a dedicated ground cable for all your electronics, electrical loads is far better to insure proper function. I've got a #4 ground run from the battery neg. postto the front of my car going to a buss strip, you can ground everything to this strip knowing it's all directly grounded to the battery.Thought of 1 other thing, have you ever had a timing lite on the engine and reved engine to the issue RPM range? If so what happens, does the timing move, do you notice the lite starts to quit flashing?