Door Hinge problems of the 1999 Chrysler Town & Country

While driving, the left rear passenger door will open on its own. All doors are locked before I start the car. This first began last June, while driving on the highway at approximately 55 mph. It happened about once every 2-3 months. Now it is happening at least 2 times a week. I am scared because I have no indication prior to its happening. A lot of the time there is no place for me to pull over to close the door. Then I am afraid the van will cut off because the open door light stays on while I am driving. This car has a little over 88,000 miles. I bought this car used 2-3 years ago.