I have seen where then engine coming to full temperature would all of a sudden loose its ability to idle. In most cases this would happen if the jets were adjusted too rich and when cold the engine not only needs this enrichening but can handle the extra enrichening due to the jets being too rich, but at full temp it becomes too rich and starts to load the plugs and then can't idle.
Jerry, when the car idles and especially during its problem idle period can you notice a sooty exhaust. Partially black smoke.
But if the idle is relatively smooth, just loses a few hundred rpms and there is no sign of sooty exhaust, AND the 'carb piston lift test' indicates that the mixture is right then I believe there is something hanging up.
I would also disconnect and plug the vacuum line to the distributor at this point. The vacuum dashpot could be moving here when its not suppose to. We also could have vacuum leaks in the intake stream that only open up when warm.
But each time I have seen this loss of idle after a few minutes of running it is usually the piston in its chamber or the jet hanging up. Especially if the choke has been pulled 'full on' because then the jet is really pulled to its maximum travel and that is where it sticks. In this case the jet needs cleaned and centered.
Dave,