As a test, next time it starts acting up immediately remove the aircleaner and look to see if the choke is closing up. If it is you should also see black smoke.
You could also be getting vapor lock on the fuel line where it runs from the fuel pump to the carb. Make sure the line isn't touching the block/head/valve cover etc., where heat coould be getting transfered to the line. May even wrap some aluminum foil around the line where it's close to a heat source for testing.
One other cause could be the vent is clogging up in the gas cap. To test for this remove the cap just when it starts acting up to see if it clears. I've had this experience on several old tractors but not in an auto, but it's the same principle.
Be careful w/the ether as an engine can get addicted to it...
