I would say it is heat related. My son and I experienced the same problem that you are experiencing with our cars. Are you using a different fuel than before? Electric or mechanical fuel pump? I would bet that oxygenated fuel is the culprit. Depending on the altitude and the mix (volitility) of the fuel it can "boil" at 140F or less. The
oil lines on our cars run right next to the mechanical fuel pump on their way to the thermostat and cooler. We installed fuel pressure gauges in our cars to see what was happening. We noticed that as the
oil temp reached 212F the pressure would drop from 6.5 lbs to 5lbs. When we stopped at a traffic light and begin to pull away at a normal traffic speed and strated to shift to second the pressure would rop to 4 lbs. The engine would sputter and cough for a few yards, the pressure would come back up to 5 and the car would run just OK. Not great but OK. The next stop light would produce the same results. The fuel was "boiling" in the pump or right before it causing it to cavitate and drop the pressure of the fuel going to the carb. In addition it was pumping hot gas to the carb.
We tried wrapping the
oil lines, pump and fuel lines with heat insulation and reflective tape. It helped but never really solved the problem. The fuel pressure would still drop. We installed electric fuel pumps and regulators. Problem solved. Pressure stays between 6 and 6.5lbs. Keep in mind that a mechanical pump sucks the fuel toward the pump, thus lowering the pressure on the fuel. Lower pressure means lower boiling point. An electric pump adds pressure to the fuel thus raising the boiling point until the fuel reaches the fuel bowl at which time the pressure drops to atmospheric. Boiling fuel in the float bowl can also cause vapor lock but it doesnt sound like your condition.
Is your fuel tank vented? It needs to be. Some guys I spoke to solved their problem by just venting the tank. You can drill a small hole in your gas cap or cut a small slot in the filler lip being careful not to get any shavings in the tank.
Good Luck