It does sound like everything has been replaced or checked carefully.
So we have to look for the odd detail if you don't believe in poltergeist.
After you set the valves, let the engine warm up and set the idle as close to 500 rpms as possible.
Attach the vacuum gauge to the wiper port.
Leave the vacuum advance connected.
Loosen the distributor port and turn the distributor until you get the highest possible reading.
Then back the distributor off 1 inch of vacuum and tighten the clamp.
it is probable that the RPM will have changed so the idle is higher. Reset the idle to 500 and repeat the timing process again.
This time the idle shouldn't change.
Timing is now optimum.
An alternative method calls for removing the vacuum advance, running the engine around 2400 rpm and setting the highest vacuum there.
I have not found this to be as reliable.
As far as new parts go, I've found so many over the years that were bad out of the box that I treat each one like it is potentially faulty.
I was asked to check a car that hesitated badly any time they stepped on the gas. It had a new carb installed.
I found the accelerator pump wouldn't work. Tore it down and finally found the passage between the float bowl and pump chamber wasn't there. There was an opening in the proper place in the bowl but it only went a couple 32nd's and ended.
You never know.
