here's one you haven't heard.... check all your calipers for dragging..... Although the above explanations would be the first place to look, but, if they are all ok and you are still experiencing the "floating" feeling, just check and confirm none of your calipers are rubbing.... It could be overheating causing the grabbing. I would still put my $$ on balljoints or FCABs, but if those are ok............
2009 335i Xdrive
JB4 and BimmerBoost Back-end flash
AA race FMIC and Diverter/Blow-off
& a few other goodies