The present work reports on a distinct and very reproducible bistable-like behavior of two defects at around E _C - 0.5 eV in MOCVD-grown GaN. The kinetics of the thermally activated transformation between the two states are analyzed in an Arrhenius model, yielding an energy barrier of 0.4 ± 0.1 eV, and a frequency factor of 10 ^6±1 s ^-1 . Depth profiles suggest that the charge state of the defects determines the observed amplitude variation. Relevant models for the observed behavior, and their shortcomings are discussed: (i) passivating properties of hydrogen, and (ii) bistable defect component(s). A proper explanation of the experimental observations represents, however, a further challenge.