A method and apparatus for determining stability of a patient's intrinsic rhythm in a cardiac pacing device. Cardiac pacing pulses are delivering to a chamber of a patient's heart by the pacing device at a first pulse amplitude. The device measures a first series of intervals between successive sensed and paced events. The device then determines whether the first measured series of intervals meets a stability criterion. Responsive to the first measured series of intervals failing to meet the stability criterion, the device determines whether the first measured series of intervals includes an interval between a delivered pacing pulse and a sensed event that is less than a defined duration. Responsive to the interval between a delivered pacing pulse and a sensed event being less than the defined duration, the device delivers cardiac pacing pulses to the chamber of a patient's heart at a second pulse amplitude and measures a second series of intervals between successive sensed and paced events and determines whether the second measured series of intervals meets the stability criterion.