Am having issues with fuel delivery system. I am on my 3rd mechanical fuel pump with vacuum. Runs great for 30 - 45 minutes, then stalls out. It appears when the fuel pump gets warm it quits pumping the fuel to the carb. If I let it cool down for approx 45 - 60 minutes, it starts & runs great again for approx 30 minutes. Any ideas on what may be going on?