This paper describes how a photomultiplier tube in a scintillation detector changes pulse height with time, when the detector is exposed to a gamma-emitting source and high dynode voltage. The change in pulse height causes a "drift" of the spectral peaks out of the preadjusted energy window, and errors in counting rate of about 20% are detected. The drift is dependent on the photon flux, the level of the high voltage, and the energy of the source. The effect was not present in all detectors studied. It is thus important to check the stability properties of a detector before use.