My garage door opens and closes by itself but seldomly.

It opens by itself but it only happens like once every couple of months but it also closed on me once.  I was unloading my groceries and it began to close so I stepped on the sensor path hopping it would stop but it kept on closing!  It only stopped when it hit my car.  By all other accounts the sensors worked perfectly every time.  It is kind of scary to go to the kitchen at 3:00 am look out the window and see your garage door opened wide.  Any ideas?  I've already ruled out a neighbor with the same remote.