Touch based rotation goes wrong after some rotation applied to model

Basically I want to rotate my 3d model using horizontal and vertical touches. For this I have written something like this type of code. But I want to restrict z rotation of 3d model so applied different code for it.

Code (CSharp):

1. Vector2 touchPosition = touch.deltaPosition;
2.     currentAngle.y += rotationSpeed.x  touchPosition.x  Time.deltaTime;
3.     currentAngle.x += rotationSpeed.y  touchPosition.y  Time.deltaTime;
4.     currentAngle.z += rotationSpeed.z * Time.deltaTime;
5.     transform.eulerAngles = currentAngle;

After applying this code, if I rotate my model 180 degree in y rotation then horizontal and vertical swipe control start working in reverse direction. If I swipe left side then model rotated over right side.

If model is rotated around 90 degree in y rotation then behaviour got changed in half way. I hope now you understand what I am trying to say.

I want functionality like, if I swipe in left direction then model gets rotated in left direction, if I swipe in right direction then model gets rotated in right direction whatever the current rotation of model. But want to restrict z rotation that is for most condition but that I have already restricted in above code.