when my bcpp was doing the yo-yo thng I remedied it by getting a century products main gear with the one way bearing and a new head ( not the whole thing just the center )
Check how much play you have in your mainshaft/rotor assembly. Grab the main gear and gently turn the rotor assembly back and forth. I bet you can move it quite a bit. You can also try using a new slightly thicker pin to hold the head on or one with a bit of a "z" bend in it to tighten up the rotor/main shaft connection. Hope that makes sense