Misfire at cruise, popping/afterfire at 3000RPM

Yes, I ran the car off of the center carb while I had the outboards removed to freshen the gaskets, etc., same problem. I've tried different springs in the secondaries, and recently I ran on just the center carb and front outboard because the rear outboard was sticky.

The misfire happens both under load and revving in the driveway. Getting back to timing -- I don't know if you've seen the posts where we talk about how the distributor only gives about 10 degrees advance by 2800, so with 14 initial, I end up all in at 24 @ 2800 RPM. I didn't think to check each cylinder with the timing light, I'll do that once it's back in the car.

This has to be a simple problem, and I'm sure someone with better troubleshooting skills would have it licked by now. It's really tempting to take it to a shop to see what they think, but I really don't know of any good Mopar shops in my area. I'm also stubborn as hell and want to figure this out.