“Spiking” is observed on the high power (∼10 MW) emitted from our 24 GHz FEL oscillator. The features of the spike are similar to those reported from our 1.5 mm wavelength FEL, but the spike FWHM is approximately 450 ps. Spikes may occur randomly or in a sequence which is not the mode-locked period of the resonator. The slippage of the FEL is varied by changing the diameter of the drift tube (cylindrical waveguide); for 17 mm diameter, the slippage should be nearly zero since the electron axial speed is the same as the wave group velocity for the TE11 mode. The absence of slippage rules out the sideband instability, but spiking occurs both with and without slippage. Comparison with a numerical model and a solitary-wave theory for spiking are presented. Measurements of the FEL spectrum are reported under conditions of multimode oscillation, feedback with a tuned reflector, and injection of a “seed” signal; the latter two methods result in a more coherent spectrum; nevertheless, spiking persists.