I have a 2000 Bonneville SSEI. A vibration / pulsating effect is felt in the steering wheel when making slight turns e.g. changing lanes, taking an exit, or simply steering away to avoid a manhole or bump. It almost seems like the wheels are wobbly. However, there is no problem / vibrations when going straight.
I have replaced my tires, rims, done wheel balancing at various sites, and have also done 4 wheel alignment. I also had the front end inspected at two different dealers, who found nothing wrong.

So far, nothing has helped. What could be the problem?

Check for binding outer cv joints, jack it up support the front of the car, not running, cut the wheels all the way see if when turning the wheel by hand you feel a binding about every 90 degrees of rotation.