AI turret rotation

Hi there
I encounter a problem when working with an AI problem.

dx = this._x -
dy = this._y -
degrees = Math.round(Math.atan2(dy,dx)*180/Math.PI)+180
distance = Math.sqrt((dx*dx)+(dy*dy))
ro = Math.round(this._rotation+this.turret._rotation)
if ((distance <= range) and (_root.stnk._alpha >= 100) and (healthC > 0)) {
  if ((degrees-ro<90 and degrees-ro>-90) or (degrees-ro>270) or (degrees-ro<-270)) {
    this.turret._rotation+= turning
  } else if ((degrees-ro<180) or (degrees-ro>-180)) {
    this.turret._rotation -= turning
  if ((ro-degrees-90<=4) and (ro-degrees-90>=-4)) {
    turning = 0
  } else {
    turning = 3

By using the above code, my turret is able to turn to the player at a constant speed. When the turret reaches the player, i.e. faces the player, however, it vibrates, or you can say occiliates.

I know the cause of this problem.
_rotation - degrees can be smaller than the turning rate of the turret so that the turret moves back again and again in order to achieve _rotation = degrees, which is impossible. But how to solve the problem?

Any help will be highly appreciated.