A mechanism has been proposed to explain the undamped spiking of the output power occurring in some multilevel maser oscillators. The phenomenon is explained in terms of the coupling between the signal frequency energy storage and the circuit properties at the pump frequency induced by the dependence of the pump level populations on the former. Approximate numerical calculations have shown that the coupling may be strong enough to render the operation of the oscillator unstable. Theoretical results obtained are in qualitative agreement with some of the observed properties of the spiking phenomenon.