If you're bouncing, you might be trying to spin too easy of a gear. There should be some tension on your pedals, even when you're "spinning." So, the next time you start bouncing, shift into a slightly harder gear and keep shifting until you stop bouncing. If you can't maintain a fast(er) cadence in that gear, it is likely something you'll have to train into.

And keep in mind that not all of us are spinners. I can maintain a cadence of 90+ rpm, but I often prefer to pedal slightly slower in a slightly harder gear. So, there's no one right cadence for everyone.