How do I remove flickering from a moving object?

I've been at this for a few weeks... boiled down to its simplest component, I want to move one single circle from the left side of the screen to the right side in a SMOOTH motion, without any flickering.

Here's my code: I instantiated a timer (timer1) and enabled in in the form's properties window and have tried values from 1-100.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApplication4
{
    public partial class Form1 : Form
    {
        private int _y;
        private int _x;

        public Form1()
        {
            InitializeComponent();
            _x = 0;
            _y = 200;
            this.SetStyle(ControlStyles.OptimizedDoubleBuffer, true);
        }

        private void Form1_Paint(object sender, PaintEventArgs e)
        {
            e.Graphics.FillEllipse(Brushes.Green, _x, _y, 30, 30);
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            _x += 1;
            Invalidate();  //ie: redraw
        }

    }
}