Ferguson TO-20 vapor locking?

I was mowing hay about a week ago with my TO-20. The temperature was in the mid to high 90's. I was using my old dyna-balance mower and running at full throttle in 1st gear. As the day got hotter the tractor would start to act like it was running out of gas. The tank was about 1/3 full. I had plenty of fuel flow at the carburetor and the elbow screen was clean. I added some more gas to the tank and the problem went away for a while. I think the gas was boiling as after a while it started to act up again. After adding more gas it ran fine until the new gas warmed up. The temperature guage was normal and the tractor didn't seem hot. I only use non-ethanol gas in all of my gas powered equipment. I'm sure the fuel is boiling, or close to it. Is there some sort of additive I could add to it to make the gas more stable or is there something else wrong?

There is no magic additive.

That sometimes happens as there is no heat shield between the engine and the tank.

Routing of the fuel line as far as practical from the exhaust helps. The line needs to be steel, not copper. Prefabbed automotive brake line works well, hand bends easily. The line should be close to the correct length as possible and all down hill, no air traps. Inline filters are also sources of trouble. They don't do well with gravity flow systems. The screen in the sediment bowl and in the carb inlet elbow are sufficient.

Good airflow is also essential. Be sure to keep the radiator fins clean, especially when mowing! Even though the engine may not be overheating, reduced air flow will increase the under hood temperature. So will a slipping fan belt.

Retarded ignition timing will raise exhaust temperatures. Be sure the distributor shaft bushings are tight, points are gapped correctly, the centrifugal advance is free, and the initial timing is correct.

And be sure the gas cap is vented. Even if it says vented, try loosening it. The vent can be clogged, or the cap mislabeled.

And finally, you can insert a sheet of insulation under the tank. It's a thin foil type insulation in the HVAC department at the big box store. Just fit it in, wire it in place.