re: Density waves
The sun is in an perturbed elliptical orbit (a rosette) about the center of mass of the galaxy (think of the line of apsides slowly rotating), but it's speed remains pretty much constant, changing only slightly because of Kepler's Law (relation of period to SM-axis). The density waves are either faster, or slower (I don't know which), so they appear to rush through the neighborhood occasionally. I think the effect you are alluding to is the repeated penetration of the galactic plane by the sun as it circles about the galaxy, caused by the wobbles in the rosette.