I don't know if this is helpful or not, but I think that it is important to remember that the function of the steering damper is not to prevent head shake, rather to damp the oscillations when they occur. Of course this is frequency dependent and the chances of predicting the correct frequency is virtually nil. So I would not expect to never have head shake just because I have a damper.
Now as for the cause of the head shake, one thing I would look at is your fork compression settings. What it sounds like happened was that when you went over the yellow lines (from you description is sounds like they may be small bumps?), you try to float the front end over them. If your forks are too stiff, instead of absorbing the bumps, the front wheel is deflected. When the wheel contacts the ground again if it is at all crooked it will oscillate about the steering axis (due to the restorative force created by the trail). I personally like to set my suspension to be very compliant, using only enough compression as necessary.