I had this almost exact thing happening to my Yukon. Do you still have the stock plastic distributor? I replaced mine with a Summit Racing one and the stumble at cruising is gone.
The gear on the original one was sharp and pointy like a knife edge, not square like new. I went under the hood to wiggle the spark plug wires at the distributor cap and the whole thing lifted up. One of the screw bosses on the plastic distributor was broken and cap was only holding on with one screw.
Vortec Distributors SUM-850098