Where is the choke pull off atMy two cents; 1. Choke pull off is not operating and opening up the choke. Choke pull off is not opening the choke plate far enough. 2. Intake manifold gasket. Had this issue many years ago. When cold the gasket is "shrunk" and leaks causing a vacuum leak. As the engine warms up the gasket swells and fills the leak and the engine operates properly until it cools again. Get a spray can of oil, start it cold and use the oil around the intake gasket area an see if the engine smooths out.