Drag rotation problem

Well.. i have a circle.. like casino roulette. All what i need is drag this circle and rotate it.

I used code snippets from other Questions.. but they dont work as i need.

I think it will work like :

OnMouseDown - get first finger/mouse position. OnMouseDrag - get current finger/mouse position and get Angle. After this, rotate object by this angle.

1 problem is when i try to rotate object for 360 degrees.. its Rewind rotate at some point..

Sorry my english.

Code i used from Mattias 1 solution.

 using UnityEngine;

using System.Collections;

public class ObjectRotator : MonoBehaviour {

 private float _sensitivity;
 private Vector3 _mouseReference;
 private Vector3 _mouseOffset;
 private Vector3 _rotation;
 private bool _isRotating;

 void Start ()
 {
    _sensitivity = 0.4f;
    _rotation = Vector3.zero;
 }

 void Update()
 {
    if(_isRotating)
    {
      // offset
      _mouseOffset = (Input.mousePosition - _mouseReference);

      // apply rotation
      _rotation.y = -(_mouseOffset.x + _mouseOffset.y) * _sensitivity;

      // rotate
      transform.Rotate(_rotation);

      // store mouse
      _mouseReference = Input.mousePosition;
    }
 }

 void OnMouseDown()
 {
    // rotating flag
    _isRotating = true;

    // store mouse
    _mouseReference = Input.mousePosition;
 }

 void OnMouseUp()
 {
    // rotating flag
    _isRotating = false;
 }

}

And there is pic.