It seems the autopilot does not maintain a consistent cruise altitude. Flying a 737-800 from LAX to DEN at FL360 with AP set to NAV, plane is constantly porpoising, with throttle bouncing from 22% to 78% every few seconds. Altitude varies from 35,700 to 36,600 MSL on the same (roughly) 20 second cycle.

No beverage service on this flight!