2004 Ford F-250

On three occasios now I have had my F250 dump about 2-3 litres tranny oil on garage floor. I initially thought it may have been due to an overfill but on two of these occasions the vehicle has done about 1500miles after a service. I have spoken to our ford dealer service department & they have never heard of the problem. At the time it dumps oil it is in the garage, overnight, has not done a long journey, the temperatures are normal ( no freezing or running in high temps) mechanic says he'll just have to pull it apart and have a look, but I would rather have a better idea if possible of what the problem might be.
Hope I have given enough info & you can help.

Thank You