The problem is quite specific to a certain Area Series setup. The problem doesn't occur with low point volumes but shows up at (tried) 10,000 points.

The following code (see comments in code) reproduce the problem when applied to a clean chart:

There are two lines just before the population loop that cause unexpected results.
===========================
        public Form1()
        {
            InitializeComponent();
 
            Area area1 = new Area(tChart1.Chart);
 
            tChart1.Aspect.View3D = false;
 
            //set a gradient colour and make the gradient visible
            area1.Gradient.StartColor = System.Drawing.Color.FromArgb(((int)(((byte)(0)))), ((int)(((byte)(128)))), ((int)(((byte)(0)))));
            area1.Gradient.Visible = true;
 
            //*******Now try commenting/uncommenting these lines one at a time
 
            //*******this line enables/diables area gradient
            //area1.AreaLines.Visible = false;
 
            //*******this line makes rendering transparent when enabled
            //area1.GradientRelative = false;
 
            double v = 0.0;
            Random r = new Random();
 
            for (int i = 0; i < 10000; i++)
            {
                v += r.NextDouble();
                area1.Add(i, v);
            }
        }
 
===========================