Left headlight goes off and on going down the road.

check electrical light connections, make sure the bulb is secure and tight, make sure the area around the lense is secured properly,make sure its the correct bulb, how long has this been happening, under what conditions do you notice the blinking light bulb, what is the weather like when this happens is it wet or windy, do you travel on unpaved areas???????, or have you recently had a stranger or novice change the bulb?? try a good visual check first, then see if something is moving that should not be moving, that could be shaking the bulb,/lense, light switch included in the checks too. headlight bulbs are sensitive to a lot of unneccessary moving, they have to be secure in the lense cover (halogen) and sealed beams brackets.