OK, I checked my wheel and actually the strange riding experience were related to some lose spokes!
I tightened them but they keep coming lose after a few km. Probably I should tighten them more, maybe use thread lock?
Have any of you experienced that?
For the wobble, it is still there, here is a video:
http://www.youtube.com/watch?v=2nZ5jnkY8YQIt is very similar to what has been reported in this topic.
It does not really affect the ride (when spokes are tight:-) but it keeps making a "wobbling noise" and is probably not to good for the gears.
I did not send any complaints to GM because I do not really see what they could do but any solutions are welcome!