First, put a vacuum guage on a port that yields true intake (not ported) vacuum. If it sags and recovers, and tweaking your idle circuit doesn't help it, you have a leak at a manifold interface.

Then, do a compression test. Adjoining low cylinders would say head gasket to me, but if you aren't seeing any bubbles in the coolant while warming, and no trace of water in the oil, it might be something else.

To me, your stumble sounds like a vacuum leak. See 79Stang351's response above. Might have a pinole gasket leak that is letting crankcase air and a little coolant seep into the end cylinders near the water passage, just enough to make the smoke.

If the engine is new, it is possible that as it heat cycles, the softer aluminum parts 'move' around slightly, causing the loss of the water seal. They need to be retorqued at least once after a hot cycle, and I've heard of people doing it routinely as a normal maintenance item.

One way to see if its a leak, take some starting fluid and CAREFULY spray around the head-to-manifold area as you advance the throttle thru the stumble. Smooth out? You got a winner.

Good luck!!!