Man, it's hard to say, but it sounds like you have a vacuum leak. That would explain the lean condition and it would cause a misfire. However, you would think that it would have the leak all the time. It is possible that the metal or plastic is expanding with heat when the engine warms up causing it to get an intermittent leak. When it is doing it at idle try spraying some carb cleaner or brake cleaner around the intake manifold. If it's got a leak it should cause the rpm to change.