How about excessive pressure in the fuel system? If the car runs for 15-20 mins, it creates a vacuum in the fuel tank, and reduces fuel flow to the engine? Just a guess.
Try testing the fuel pressure? Monitor it from the time the engine starts until it stalls. At least, that's the next step that I'd do.
Keep in mind I'm only a hobbyist mechanic. So my opinions aren't really worth much. But every now and then, I do say something valuable.
