Every since I purchased my Mitsubishi Endeavor I have had to replace the headlight bulbs about every 3 months. Don't know what causes them to blow. Was told by mechanic to put some lubricating grease on bulb and it would keep them from blowing out but that did not work. Still every 3 months I have to replace bulbs. One side will go out then about 3 months later the other side will go out. What can cause this to keep happening?