HT crank pinch bolts need to be tightened near simultaneously, while many people screw that up and tighten them one at a time. Loosen them both up, adjust the preload, and alternate until they come up to (I think) 14Nm. If that doesn't make the BB feel smooth and without play, you have something worn or broken.

The preload screw not only can't cause a crank to come loose, but the tightened crank keeps the preload screw from turning.