I know this question is a bit old... but I thought I would throw my 2 cents out there anyways. I have observed the G1000 oscillating up and down if you are exceeding the maximum speed of the aircraft. Since there is no auto throttle, pulling the nose up is the only way the autopilot can reduce airspeed. Of course, you are now flying above the assigned altitude, which will push the nose down in an attempt to get back to the assigned flight level. This process will repeat. Pulling back on the throttles will allow the autopilot to regain stable flight.