This study investigates an apparent shear flow instability observed in a stably stratified boundary layer on 21 October 1999 during the Cooperative Atmosphere-Surface Exchange Study (CASES-99) in south-central Kansas. A scanning Doppler lidar captured the spatial structure and evolution of the short-lived instability, and high-rate in-situ sensors mounted on a nearby 60-m tower provided stability and turbulence data with excellent vertical resolution. The event occurred shortly after sunset during an evening transition period in which the flow aloft (>100 m AGL) underwent an oscillatory motion with a period of roughly 30 min. The oscillations were observed between roughly 0000 and 0300 UTC, and were evident in time series of the low-level jet (LLJ) height derived from Doppler lidar data. Modulation of the vertical shear below 100 m AGL periodically produced conditions conducive to the formation of shear instability. Between 0000 and 0300 UTC turbulence near the surface was generated in quasi-periodic bursts which appear to correlate with the oscillations in the LLJ height. The lidar recorded the formation, evolution and decay of a shear flow instability during one cycle of this oscillation. This study presents these observations and provides estimates of the basic wave parameters associated with the instability and the larger scale forcing that produced it.