Make a sphere appear to roll down a slope

Unity Engine
1

I am trying to make a 3D sphere appear to roll down a slope.

For reasons I don’t think I need to go into, I am not using any other 3D objects, gravity or rigid bodies etc.

Basically the sphere is one 3D obect among a whole lot of sprites.

The sphere was created in Blender so the axes are arse about - to make it move in the y direction in Unity I need to manipulate the sphere’s z axis.

The point is that I can make the ball either roll in forward direction or slide down the slope, but I can’t figure out how to do both at the same time.

gameObject.transform.RotateAround(m_collider.bounds.center, Vector3.back, 5.0f);
gameObject.transform.Translate(GetTransVector());

If I comment out the first line, the sphere slides down the slope nicely.
If I comment out the second line the sphere rotates about its centre nicely.
But both together then the sphere rotates in a circle on the spot.

Any suggestions?

    private Vector3 GetTransVector()
    {
        if (gameObject.transform.localPosition.x >= (m_gameObjectRecentBall1.transform.localPosition.x))
        {
            m_bStartRoll = false;
            DoShow (false);
            gameObject.transform.localPosition = m_vect3OriginalPos;
            m_vect3DeltaTrans.x = m_fDeltaX;
            m_vect3DeltaTrans.y = 0.001f;
            m_vect3DeltaTrans.z = m_fDeltaY;
            m_parameters1.New (BingoMessageIDEnum.MSGID_NEW_BALL_SPRITE, m_spriteBall);
            SendMessage ("LastThreeBalls", BingoMessageIDEnum.MSGID_NEW_BALL_SPRITE, m_parameters1);
        }
        else if (gameObject.transform.localPosition.x >= (m_gameObjectRecentBall1.transform.localPosition.x - 0.95f))
            m_vect3DeltaTrans.z = 0.0f;

        return m_vect3DeltaTrans;
    }

    protected override void Update()
    {
        if (!IsHidden() && m_bStartRoll)
        {
            gameObject.transform.RotateAround(m_collider.bounds.center, Vector3.back, 5.0f);
            gameObject.transform.Translate(GetTransVector());
        }
    }
2

The rest of the code if needed:

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

public class RollerBallManager: McCormickMonoBehaviourBase
{
    private GameObject m_gameObjectRecentBall1 = null;
    private Vector3 m_vect3OriginalPos = new Vector3();
    private float m_fDeltaY = 0.004f, m_fDeltaX = 0.02f;
    private Vector3 m_vect3DeltaTrans = new Vector3(0, 0, 0);
    private bool m_bStartRoll = false;
    private List<Texture2D> m_arrayTextureBalls = new List<Texture2D>();
    private Sprite m_spriteBall = null;
    private SphereCollider m_collider = null;

    protected override void Start()
    {
        string strTexturePath = "Textures/Ball";
        Texture2D texture = null;

        for (int nI = 1; nI <= 75; nI++)
        {
            texture = Resources.Load<Texture2D>(strTexturePath + nI.ToString("00"));
            m_arrayTextureBalls.Add(texture);
        }
        m_gameObjectRecentBall1 = GameObject.Find("RecentBall1");
        m_vect3OriginalPos = gameObject.transform.localPosition;
        m_vect3DeltaTrans.x = m_fDeltaX;
        m_vect3DeltaTrans.y = 0.0f;
        m_vect3DeltaTrans.z = m_fDeltaY;
        m_collider = GetComponent<SphereCollider>();
        DoShow(false);
    }

    private Vector3 GetTransVector()
    {
        if (gameObject.transform.localPosition.x >= (m_gameObjectRecentBall1.transform.localPosition.x))
        {
            m_bStartRoll = false;
            DoShow (false);
            gameObject.transform.localPosition = m_vect3OriginalPos;
            m_vect3DeltaTrans.x = m_fDeltaX;
            m_vect3DeltaTrans.y = 0.001f;
            m_vect3DeltaTrans.z = m_fDeltaY;
            m_parameters1.New (BingoMessageIDEnum.MSGID_NEW_BALL_SPRITE, m_spriteBall);
            SendMessage ("LastThreeBalls", BingoMessageIDEnum.MSGID_NEW_BALL_SPRITE, m_parameters1);
        }
        else if (gameObject.transform.localPosition.x >= (m_gameObjectRecentBall1.transform.localPosition.x - 0.95f))
            m_vect3DeltaTrans.z = 0.0f;

        return m_vect3DeltaTrans;
    }

    protected override void Update()
    {
        if (!IsHidden() && m_bStartRoll)
        {
            gameObject.transform.RotateAround(m_collider.bounds.center, Vector3.back, 5.0f);
            gameObject.transform.Translate(GetTransVector());
        }
    }

    private Texture2D GetBallTexture(int nBallNum)
    {
        Texture2D textBall = m_arrayTextureBalls[nBallNum - 1];

        return textBall;
    }

    protected override void DoReceiveMessage(Parameters p)
    {
        if (p.m_nMsgID == BingoMessageIDEnum.MSGID_START_ROLL)
        {
            m_spriteBall = (Sprite)p.m_objParam1;
            Texture2D textBall = GetBallTexture((int)p.m_objParam2);
            Material material = GetComponent<Renderer>().material;
            material.SetTexture("_MainTex", textBall);
            m_bStartRoll = true;
            DoShow(true);
        }
        base.DoReceiveMessage(p);
    }
}