A broadly observed phenomenon in experiments on molecular junctions is time dependent switching of the tunneling current. In many cases such behavior involves different current states which are attributed to the transfer of single atoms or functional groups in a molecule between different stable configurations. We describe here the investigation of the current switching observed in a molecular junction formed by a PTCDA molecule between an STM tip and an Ag(111) surface, which is believed to be due to the carboxylic oxygen atom switching between the surface and the tip. We use a generalized version of a model developed in 1997 by Gao et al. to investigate the results observed in these experiments. The distribution of the switching events measured in the experiments shows a power law dependence for small bias voltages, whereas for higher voltages it first saturates and then drops again. Compared to Gao's model, which can only describe the rapid increase of the switching events at a certain onset voltage, our extended model can be used to characterize the whole distribution relating the different behavior to the changes in the potential by the applied bias.