What sort of shifter are you using? When I was in QC at T&C Livonia, we had a spate of trouble on C6 transmissions with the park function. It was due to the shifter not pushing the park rod far enough to completely get the pawl into position, or not holding it there. If the rod backs off, so will the pawl.
Another possibility is that the park mechanism has been 'chewed' by putting the trans in park when the car isn't completely stopped, and causing it to 'ratchet' down to zero. After a few times, the detents aren't all they should be.
Just a few possibilities.
KS