The output characteristics of a high-intensity laser oscillator are examined by means of a semiclassical density-matrix approach. Unlike previous investigations the time derivatives of the field and material quantities are retained, and a basic instability in the semiclassical laser equations is revealed. This instability accounts for the recently reported spontaneous pulsations in xenon laser oscillators. The pulsations can be interpreted as a consequence of spectral holeburning in inhomogeneously broadened lasers, and stability criteria are derived for both standing-wave and traveling-wave oscillators. In simplest terms the pulsations should occur in any inhomogeneously broadened laser where the product of the homogeneous linewidth and the cavity lifetime is less than unity.