The mechanism that causes a periodic combustion oscillation in a Lean Premixed Prevaporized (LPP) Combustor was experimentally investigated. The combustor was run with high flowrates of preheated air at elevated presures, with Jet-A as the fuel, and with a realistic fuel injector. The injector provides LPP combustion and features a stable diffusion flame as a pilot surrounded by a premixed main flame. The combustor was stable over a wide range of conditions, but for off-design conditions oscillations associated with unsteady flashback and liftoff of the flame base were observed. These low-frequency combustor instabilities were a strong function of a global velocity gradient (U/d), which is analogous to the U/d gradient used to characterize stability limits of Bunsen flames. PIV data quantified the location and strength of shear-layer vortex shedding. The main and pilot flames were visualized using PLIF images of formaldehyde. These images showed that for design conditions the pilot flame is very stable and that the main premixed flame exists in the mixing layer. It was seen that as the global gradient U/d decreases, the local velocity gradient in the mixing layer also decreases, making the flame more susceptible to perturbations from the vortex-shedding. It is proposed that the unsteadiness in the combustion can be explained using classic theories of premixed flame flashback and blowout that was developed for Bunsen burners.