线条不是每次都绘制到图片框中

当我的代码只运行了大约1/3的时间时，它会正确地绘制，其余的时间则不会绘制一些线条。

Pen pen = new Pen(Color.Black);
        Graphics graphics = MainFrm.HangmanImage.CreateGraphics();
        switch (NoOfGuesses)
        {
            case 1://Bottom left leg
                graphics.DrawLine(pen, 15, 60, 40, 60);
                break;
            case 2://Bottom right leg
                graphics.DrawLine(pen, 40, 60, 55, 60);
                break;
            case 3://Middle stock
                graphics.DrawLine(pen, 34, 20, 34, 60);
                break;
            case 4: //Top line
                graphics.DrawLine(pen, 34, 20, 55, 20);
                break;
            case 5: //Rope
                graphics.DrawLine(pen, 55, 20, 55, 25);
                break;
            case 6: //Head
                graphics.DrawEllipse(pen, (new Rectangle { X=52, Y=25, Height=5, Width=5 }));
                break;
            case 7: //Body
                graphics.DrawLine(pen, 55, 30, 55, 35);
                break;
        }
        pen.Dispose();
        graphics.Dispose();