I suspect the lug nuts that loosen relatively soon after being torqued properly has to do with how centered the wheel was and the tightening pattern used. If you just shove the wheel on and the torque all of the nuts in a circle (especially with an air impact wrench) it's likely the wheel ends up very slightly shifted on the lug tapers. After driving some it moves a little, freeing up enough space for the torque to decrease on the nuts.
If you torque in a criscross pattern first half way, then all the way I don't see how they can loosen up as easily.
