Inspect all your fuel lines, on second thought, replace all your fuel lines (about $5 at any auto parts store). As the line ages, they develop micro cracks. Flexing the lines will result in air drawn into the fuel supply system. You may have a damaged "Pulse" line (the line from the fuel pump to the engine) When this line starts sucking air, the fuel pump can no longer draw gas from the tank.
You can check the fuel supply system by disconnecting the line from fuel pump to the carb. at the carb end. With the key on, step down on the gas peddel so the starter will engage the engine. If you are getting only a small "dribble" out of the line, your fuel supply system need help,. You should be getting a strong amount of fuel from the line **Caution** No SMOKING during the test