If you can get alcohol free fuel in your area run your tank down to almost empty fill it up with alcohol free, drive it down and refill again then go testing.
My stock 67 273 2bbl would do the same thing after ideling in a drive through line in the summer. Now with alcohol free doesn't do it anymore.
If you let the car idel after it is hot, does your idel speed start to decline, then all of a sudden start to return? Maybe 1 minute between cycles.
This what I encountered, just ideling.
Imagine the filter being empty as you punch it, the bowels are still full enough to run the car for a few seconds and at the point the float gets low enough to allow new fuel into the bowl, it has to vent all the fuel vapors first and then refill the filter and bowl.
https://photos.app.goo.gl/S5t4cX7FXFmRZhSS6
