Try pulling the hose from the vacuum canister on the side of the distributor. Plug the hose end, perhaps with a small sheet metal screw. I found that even though I had the advance set correctly, the cacuum was causing enough retard of advance (if that makes any sense) to cause some stalling.

One other thought. When the engine is cold, the auxiliary air slide is open and sending lots of air. As the engine warms up, the slide closes and air is then entirely regulated by the idle screw. So, maybe you are getting enough air when the engine is cold but, after warm up the idle is set way too low.