I've had a very similar problem on my Vectra (still having it). after putting up with it for several months Eventally something broke, now when I turn left the indicator comes on and doesn't auto turn off. Very Annoying. It also had a habit of sticking, but since whatever it was broke it doesn't stick any more.
Please let me know if you get it resolved and what it is, I think I will have to get mine fixed one day.