My son and I both have 427R engines in our SPF cars. We both had the same problem, and resloved it. Trust me and the others who have advised you on the fixes. We had the same intermitent stalling or poor performance on 75 degree days here in AZ. We have diagnosed this problem for other enthusiasts on four other carbed cars here in AZ in Cobras and Rods.
1. Check out the gas cap situation. Make sure the tank is vented properly. Mine was not, my sons was.
2. It is most likely vapor lock even with low outside temps. We have discovered that some of the oxygenated fuel in AZ boil at 140 degrees. The fuel in your lines between the mechanical fuel pump and carb (in the fuel log) is boiling when you stop for a few minutes to a half an hour. It is also boiling in the float bowls. The location of the mechanical fuel pump is also a problem when you consider the
oil lines to the
oil cooler are running right next to the fuel pump. When that
oil temp is up about 200 plus degrees, it transfers the heat to the mechanical fuel pump and thereby the fuel, the fuel begins to boil and then the pump cavitates and starts to push air instead of fuel. We installed an inside cab fuel pressure gauge as we diagnosed the problem and could easily watch the fuel pressure drop as the engine heated up and the oil heated up. The pressure would fall below 4lbs and the engine would begin to puke and/or stall. Typical vapor lock. If in doubt, install a fuel pressure gauge in the cab and watch the fuel pressure. If it drops below 5lbs you will have problems. The Holley on your car needs at least 6 lbs to run right. Holley may tell you differently, but that's what we discovered
The Fix: Install an electric fuel pump and regulator. Install a Holley spacer and Holley heat sheild beneath the carb. This will take care of about 98% of the problem. The added height will clear the underside of your hood scoop.
If you want the best fix, do all of the above but add a return fuel line to the tank. We did one of our cars without the return line and one car with. The car with the return line did not experience any problems while the one without the return line would still experience vapor lock, but rarely. We completed the return line install on that car as well. Zero problems on both cars.
But if you really want to get your engine running well, ditch the Holley that Roush provides and go with a Pro-Systems Carb. Outstanding Performance. Runs better, idles smoother no hassle to set it up. The Roush/Holley has been tinkered with at Roush and is difficult to get running perfect all of the time, especially with oxygenated fuels.
PM me if you want the complete specs of what my son and I did. I would be happy to supply them to you.