I agree, ignition problems are the most common. Spark can leak between crossed wires, and the MSD may aggravate that. Thick silicone secondary wires help prevent that. Carbon tracking and even moisture inside the distributor cap can do that but I'm sure you are using brand new stuff. A very lean carburetor will pop back under load. You claim no vacuum leaks but that causes a lean carb. Internal leak? Then there are the valves that may not be closing all the way. Are you using Pontiac jam nuts, or poly-locks? If poly-locks the only way to set them is with the engine idling so you can absolutely find zero lash when they clatter. I have seen guys set them statically, leave them too tight, and end up with valves that stay open resulting in backfires. They don't want to adjust them running and get an oil shampoo because their radical cam requires 1200 rpm idle and that gets the oil pressure so high the pushrods squirt like super-soakers. They used to sell little clips that covered the squirt holes while you adjusted the poly-locks. If you used Pontiac jam nuts, torqued to 20ft lbs, then the lifters are self-adjusting and you can rule that out. Solid lifters? Same as pol-locks- could be too tight. But it's OK to set those cold. That's all I can think of off hand.