It would be interesting to know where exactly it is during the back swing that the bobbing occurs. Start-up, back swing, top, end? Personally, I hold the record for swaying and bobbing, and I don't like it when someone else is encroaching on my record, so, it's important to get you fixed as soon as possible. I've asked my wife to watch me swing and to tell me when I start to bob, and so far I have address and finish pretty well under control.
7-16 "Actually, the primary function of knee action-as with waist bend-is to maintain a motionless head during the stroke".