Welcome to the wonderful world of the BCP. Erratic flight characteristics such as these are simply unavoidable with it's design. It is possible to get it 'dialed in' fairly well for a few flights, but then the trouble returns. Best thing to do is to get a better heli IMO...

Once I got my Falcon 3D ($279 RTF from hobby-lobby.com) and put a few flights on it, my BCP went on the shelf forever...


But for the yo-yoing, If the collective is not sticking, blade tracking is OK, and you are sure there are no stripped teeth in the servos, then check the head dampening. If it's too loose, it will do this badly. If it is loose, you can add a shim or two in front of the o-rings to 'clean it up', but like I said above, it will probably return after a few flights...