mine has done it twice....both times was a bad shifter...replaced under warranty....but now i know how to fix it...if you remove the shifter knob and then the cover with the PRNDL you will find a little switch there...its a bronze or gold looking strip....you might be able to fix it...just sounds like its losing its tension...with shifter in L and the ebrake on so the car dont happen to roll....push on the tab up so that u give it a lil more tension (careful dont break it) and it might last a bit longer...or you could just go in and get it replaced...
or just live with it and use the the switch underneath the steering column to release the key...if you look in your owners manual it tells you where this key release is located under the steering wheel. basically its located right behind a lil 1 inch panel underneath the steering