DataGridView 表头合并问题

通过重写DataGridView的OnCellPainting 重写绘制表头  但是拖动滚动条的时候 列上面的文字就乱掉了 可能是因为性能问题  因为在不听的调用这个重写方法我该怎么搞 高手给个解决方案代码如下：
 public partial class RowMergeView : DataGridView
    {
        #region 构造函数
        public RowMergeView()
        {
            InitializeComponent();
        }
        #endregion
        #region 重写的事件
        protected override void OnPaint(PaintEventArgs pe)
        {
            // TODO: 在此处添加自定义绘制代码            // 调用基类 OnPaint
            base.OnPaint(pe);
        }
        protected override void OnCellPainting(DataGridViewCellPaintingEventArgs e)
        {
            try
            {
                if (e.RowIndex > -1 && e.ColumnIndex > -1)
                {
                    DrawCell(e);
                }
                else
                {
                    //二维表头
                    if (e.RowIndex == -1)
                    {
                        if (SpanRows.ContainsKey(e.ColumnIndex)) //被合并的列
                        {
                            //画边框
                            Graphics g = e.Graphics;
                            e.Paint(e.CellBounds, DataGridViewPaintParts.Background | DataGridViewPaintParts.Border);
                            
                            int left = e.CellBounds.Left, top = e.CellBounds.Top + 2,
                            right = e.CellBounds.Right, bottom = e.CellBounds.Bottom;                            switch (SpanRows[e.ColumnIndex].Position)
                            {
                                case 1:
                                    left +=2;
                                    break;
                                case 2:
                                    break;
                                case 3:
                                    right -= 2;
                                    break;
                            }                            //画上半部分底色
                            g.FillRectangle(new SolidBrush(this._mergecolumnheaderbackcolor), left, top,right - left, (bottom - top) / 2);                            //画中线
                            g.DrawLine(new Pen(this.GridColor), left, (top + bottom) / 2,right, (top + bottom) / 2);                            //写小标题
                            StringFormat sf = new StringFormat();
                            sf.Alignment = StringAlignment.Center;
                            sf.LineAlignment = StringAlignment.Center;                            g.DrawString(e.Value + "", e.CellStyle.Font, Brushes.Black,
                            new Rectangle(left, (top + bottom) / 2, right - left, (bottom - top) / 2), sf);
                            left = this.GetColumnDisplayRectangle(SpanRows[e.ColumnIndex].Left, true).Left - 2;                            if (left < 0) left = this.GetCellDisplayRectangle(-1, -1, true).Width;
                            right = this.GetColumnDisplayRectangle(SpanRows[e.ColumnIndex].Right, true).Right - 2;
                            if (right < 0) right = this.Width;
                         
                            g.DrawString(SpanRows[e.ColumnIndex].Text, e.CellStyle.Font, Brushes.Black,
                                new Rectangle(left, top, right - left, (bottom - top) / 2), sf);
                                e.Handled = true;
                        }
                    }
                }
                base.OnCellPainting(e);
            }
            catch
            { }
        }