What year & model? Do you ever notice moisture or water in then after washing or a rainy day? I have the same issue on my 07 3.0si. It is due to weak sealing around the light housing that causes water or moisture to go in and cause the malfunction. I plan to switch it with a new light housing which was updated to include a better gasket sealing around it.
http://youtu.be/KGZ3GxH0Fs0