2002 X-type 3.0 front blinkers go out intermittently
 

Very frustrating. Both blinkers in front occasionally stop working until I unplug the bulb and put it back in. They work for several weeks and then one or both will stop working again. Back blinkers are fine. I have cleaned the sockets and lubricated appropriately but still seem to have the occasional problem. Anyone have any suggesttions?