A few things I would check that have not been mentioned:
1) I would check the i tegrity of the floars in your carburetor. It would not hurt to change the needle&seat assemblies as a maintenance item.
2) I would 2x check for a vacuum leak. Verify the base plate to main body screws are tight, etc. Not all vacuum leaks come from the manifold.
My guess is this is not ignition related. One way to peoce that is to ensure there is not output from the MSD is switched off. You could test that with the white points trigger wire and a spark plug plugged into the coil wire.
The floats are good. The float level is steady as a rock. The MSD is something that I haven't thought of. If I recall, with my old 6AL I didn't have this problem. Maybe explain in detail your recommendation for checking it.
No vacuum leaks. I checked that.
