I'm one of those low cadence smasher girls too, about 60-75 rpm. I read somewhere that I'm more comfortable slow because I'm not as aerobically fit as I could be. I dunno, it's how I ride and how I raced. Since I want to keep my knees in some kind of functionality I have been working on my peddaling technique, making it more round and that seems to be bringing my cadence up, too. I do notice that I am more likely to shift to an easier gear now when my legs say to, not that I've ever noticed knee pain specifically.

Could saddle height have something to do with the bouncing, if you are already peddalling a nice smooth circle?

Just a work in progress!