A self-sustained fully-passive flapping foil hydrokinetic turbine prototype is investigated experimentally. The foil was undergoing elastically constrained oscillations in pitch and heave in water at Re = 21000. The prototype was subjected to inflow disturbances in the form of symmetric vortices from a wake of an actuated oscillating foil positioned upstream of the test foil. The effects of the periodic inflow disturbances on the fully-passive turbine performance were characterized using measurements of its kinematics, power coefficient and the flow structures in the near-wake region. Stable operation of the turbine was observed below a threshold reduced frequency of the incoming vortices. Above the threshold frequency, intermittent resonance with the upstream flow disturbance resulted in unstable oscillations of the turbine. The range of the frequencies of the upstream disturbance that were beneficial to the power performance of the fully-passive turbine was not consistent with the frequencies of the turbine oscillations in a uniform stream with equivalent momentum flux. Peak power extraction could be obtained in a narrow range of kinematics of the upstream oscillating-foil.