Where I live there is a garage called "Stephenson Steering Specialists". Guess what their are good at.
If you can find such a business near where you live, you should try them for a diagnosis. Especially since the dealer didn't solve it first try.
Your problem sounds just like one I had 40 years ago with my '73 Z28 Camaro except I found the problem most noticeable at higher speeds (but it was sharp turns (which are done while driving at low speeds) which trigger it. Stephenson's diagnosed it with a 10 minute road test followed by a few minutes on the hoist, as an idler arm bearing that was binding. Most modern GM cars have rack and pinion steering so no idler arm. But I would suppose that any bearing which is part of steering such as the steering knuckles that your dealer lubricated could be the cause. The steering rack has slide type bearings which I would expect could also be the cause. My problem was intermittent like yours also. The reason it was intermittent was because the binding bearing still moves when the steering is turned but not quite all the way and tries to resist the movement of the steering. So if your last sharp turn was to the right, the car will pull to the right. If your next sharp turn is the left then the car will now pull to the left. If you then turn to the right again but only half way to the steering limit, the car may now steer straight or at least straighter.
And of course I am not a steering expert so the cause of your problem could be different. but I think the dealer should have at least road tested when they were finished as I just described.
Best wishes with this.
P.S. One other difference with my experience was that I knew my problem was inconsistent when I took it to the garage and told them so. So perhaps your dealer deserves the benefit of the doubt for not fixing the problem first try.