Engine Stuttering Options

[GulfRacer]

I'm hoping some collective wisdom can help me through a frustrating problem. I have 1974 Bumblebee that I've owned for about 6 months. The 2.0 was rebuilt to a 2056 shortly before I purchased it. It has a Raby 9590 camshaft, original D-Jet and 123 distributor. When I first got the car, it was running great. It has since developed an intermittent stutter/bucking. It occurs most commonly around 3000 RPM when cruising, but once it happens, it continues when applying throttle. It doesn't usually occur until the engine has warmed up - the first several minutes of the drive are usually good.

I replaced the throttle sensor board which did not solve the problem. I disconnected the throttle sensor, and the problem is still present. I replaced the fuel injection harnesss which did not help. The fuel injectors are all new with the engine rebuild, but cylinder 3 injector stopped firing a few months ago, and I replaced it with a new one.

I'm at a loss as to where to go next. Any advice would be greatly appreciated.

Jason

[GulfRacer]

I still haven't sorted this problem out, and since the original post, the problem has become more consistent which at least makes it a little easier to troubleshoot. The problem no longer seems to be heat related or at a specific RPM, and the car is pretty much undrivable now.

Since the last post, I replaced the ignition wiring harness and redid the spade connectors on the 123 distributor to the coil. I replaced the distributor cap (carbon center piece worn away) and rotor. I replaced the CHT sensor. I think as a separate issue last week, the fuel pump wouldn't come on, and I traced that to a bad relay board. I was hoping maybe that was the problem for the poor running. A borrowed relay board allowed me to get the car started, but the rough running (and backfiring) is still there. Fuel pressure remains steady at 28-29. MPS holds vacuum. I've disconnected each fuel injector, one at a time, and idle drops like you'd expect. Rough running persists when firing on 3 cylinders. Plug wires all look fine and firmly attached. All ground connections are good

Any other suggestions. Remaining possibilities that I can think of are a bad ECU or bad 123 distributor. What am I missing?