Drag - drop custom SKSpriteNode position issue - swift

Question

I'm my game I'm using a custom SKSpriteNode which the player are going to be able to drag and drop at a position of his/hers desire.
My custom node is made of this code in file DraggableNode.swift:
class DraggableNode : SKSpriteNode {
var nodeSelected : SKNode?
init() {
let texture = SKTexture(imageNamed: "draggableNode_1")
super.init(texture: texture, color: UIColor.white, size: texture.size())
self.isUserInteractionEnabled = true
self.zPosition = 1
}
required init?(coder aDecoder: NSCoder) {
super.init(coder: aDecoder)
}
override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent?) {
if let touch = touches.first {
let location = touch.location(in: self)
let touchedNodes = self.nodes(at: location)
for node in touchedNodes.reversed() {
nodeSelected = node
}
}
}
override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent?) {
if let touch = touches.first, let node = self.nodeSelected {
let touchLocation = touch.location(in: self)
node.position = touchLocation
}
}
override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent?) {
nodeSelected = nil
}
override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent?) {
nodeSelected = nil
}
}
In my GameScene.swift I add the custom node in this way:
let node = DraggableNode()
node.position = CGPoint(x: randomX, y: randomY)
self.addChild(node)
Now, the problem is that when I start to drag the node it's all jump and is placed a total random place - sometimes even out of the screenarea.
I wonder if the problem consists of the fact that it's a custom node inherited from a SKSpriteNode?
If anyone can point me in the right direction, I'd be a happy man :-)