My '75 Honda 750 runs perfect For about 20 min... FIXED! :)

My '75 Honda 750 runs perfect For about 20 minutes and then starts running less than perfect, missing on at least one cylinder, but still drivable. If I let it cool off for 30 minutes or so it runs perfect again for about 20 minutes then the same missing again on some cylinders.

I've just completely torn down the carbs for complete cleaning over that last week, rebuilt them, and put them back on and still the same issue.

I just installed all brand new plugs, one heat range cooler, gapped at 26 thou, and still the same issue.

It has fresh premium grade fuel for the last couple tanks, with a bit of Lucas fuel treatment, and still the same issue.

'75 Honda 750, Stock engine, stock jets, 4-2 exhaust.

Again, if I let it sit for 30 minutes or so after acting up, it runs perfect for about 20 minutes then starts missing so I'm thinking something is getting up to operating temp and then unhappy

OR could it be taking that long to get fuel starved and then resting for a few minutes lets the bowls fill back up??? (hmmm) Again, this is happening before AND after the carb cleaning/adjustment. This is happening while driving stop-and-go around the neighborhood with stop lights, signs, etc... so it's not high rpm driving.

Ideas please?