I would suggest increasing the frame rate (to 30) if you haven't done so, to add smoothness, and continuing to play with the kMinSpeed and kMaxSpeed numbers. Somewhere between too slow and too fast there must be a 'just right' hiding.

Also, you might want to turn off the 'sway' stuff till you get the vertical motion right (the adjustments to _y), then work on the sway separately.

It's best to tweak one number at a time.