my front left wheel. when im driving starts to lock up?

when i am driving my front left wheel feels like it is locking up 4 a very brief second but will continue to do this for as long as i drive the car. my local garages cant identafi a problem but i knoe thare is 1 b ut have no idear wat it is lol..... any idears will help.... and my car is a citroen saxo 2001....