As Dave3913 elluded to, see sawing or front to back, is either low head speed or tight head dampening. Check out your see saw to be sure it is free in movement as this can cause oscilating, moving around in a circle if veiwed from above, which can look like bobbing at an eye sight level.
For our info, can you give us an idea of your head speeds? If no tach., pinion and motor kv. as well as throttle curve?