Randomly locking doors....?

I have a 2004 Pontiac Grand Prix with approx. 47,000 miles on it. It recently started to randomly lock while i am driving. It locks when i put into Drive and will stay locked but will make the clicking sounds as if its re-locking itself while i'm driving. This will usually happen about 25 mph. I have,on occasion, had the driver side door stay locked when i unlock the doors. i would manually have to unlock it. what could be the cause and how can i fix it?