Ok,

After some testings, i found what is causing all my forms weird behaviors, it looks like there is a bug when playing with the opacity level of an MDI Parent. Here's the repro code:

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.Data;

namespace WindowsApplication1
{
	/// <summary>
	/// Summary description for Form1.
	/// </summary>
	public class Form1 : System.Windows.Forms.Form
	{
		private System.Windows.Forms.MainMenu mainMenu1;
		private System.Windows.Forms.MenuItem menuItem1;
		private System.Windows.Forms.MenuItem menuItem2;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;

		public Form1()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();
                        ///// reset the opacity to 1 here
                        ///// usually i would use a timer to do so
                        ///// this line is not really the problem
                        ///// and can stay here
			this.Opacity = 1d;
			//
			// TODO: Add any constructor code after InitializeComponent call
			//
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if (components != null) 
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.mainMenu1 = new System.Windows.Forms.MainMenu();
			this.menuItem1 = new System.Windows.Forms.MenuItem();
			this.menuItem2 = new System.Windows.Forms.MenuItem();
			// 
			// mainMenu1
			// 
			this.mainMenu1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
																					  this.menuItem1});
			// 
			// menuItem1
			// 
			this.menuItem1.Index = 0;
			this.menuItem1.MenuItems.AddRange(new System.Windows.Forms.MenuItem[] {
																					  this.menuItem2});
			this.menuItem1.Text = "File";
			// 
			// menuItem2
			// 
			this.menuItem2.Index = 0;
			this.menuItem2.Text = "Test form";
			this.menuItem2.Click += new System.EventHandler(this.menuItem2_Click);
			// 
			// Form1
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(640, 481);
			this.IsMdiContainer = true;
			this.Menu = this.mainMenu1;
			this.Name = "Form1";

///////////Removing this line will fix the problem /////////////////////
			this.Opacity = 0;
////////////////////////////////////////////////////////////////////////
			this.Text = "Form1";

		}
		#endregion

		/// <summary>
		/// The main entry point for the application.
		/// </summary>
		[STAThread]
		static void Main() 
		{
			Application.EnableVisualStyles();
			Application.DoEvents();
			Application.Run(new Form1());

		}

		private void menuItem2_Click(object sender, System.EventArgs e)
		{
			Form2 frm = new Form2();
			frm.MdiParent = this;
			frm.Show();
		}
	}
	public class Form2 : System.Windows.Forms.Form
	{
		private System.Windows.Forms.Label label1;
		private System.Windows.Forms.TextBox textBox1;
		private System.Windows.Forms.ListBox listBox1;
		private System.Windows.Forms.TextBox textBox2;
		private System.Windows.Forms.TextBox textBox3;
		private System.Windows.Forms.TextBox textBox4;
		private System.Windows.Forms.TextBox textBox5;
		private System.Windows.Forms.TextBox textBox6;
		/// <summary>
		/// Required designer variable.
		/// </summary>
		private System.ComponentModel.Container components = null;

		public Form2()
		{
			//
			// Required for Windows Form Designer support
			//
			InitializeComponent();

			//
			// TODO: Add any constructor code after InitializeComponent call
			//
		}

		/// <summary>
		/// Clean up any resources being used.
		/// </summary>
		protected override void Dispose( bool disposing )
		{
			if( disposing )
			{
				if(components != null)
				{
					components.Dispose();
				}
			}
			base.Dispose( disposing );
		}

		#region Windows Form Designer generated code
		/// <summary>
		/// Required method for Designer support - do not modify
		/// the contents of this method with the code editor.
		/// </summary>
		private void InitializeComponent()
		{
			this.label1 = new System.Windows.Forms.Label();
			this.textBox1 = new System.Windows.Forms.TextBox();
			this.listBox1 = new System.Windows.Forms.ListBox();
			this.textBox2 = new System.Windows.Forms.TextBox();
			this.textBox3 = new System.Windows.Forms.TextBox();
			this.textBox4 = new System.Windows.Forms.TextBox();
			this.textBox5 = new System.Windows.Forms.TextBox();
			this.textBox6 = new System.Windows.Forms.TextBox();
			this.SuspendLayout();
			// 
			// label1
			// 
			this.label1.Location = new System.Drawing.Point(8, 16);
			this.label1.Name = "label1";
			this.label1.TabIndex = 0;
			this.label1.Text = "A label";
			// 
			// textBox1
			// 
			this.textBox1.Location = new System.Drawing.Point(8, 40);
			this.textBox1.Name = "textBox1";
			this.textBox1.TabIndex = 1;
			this.textBox1.Text = "textBox1";
			// 
			// listBox1
			// 
			this.listBox1.Location = new System.Drawing.Point(8, 128);
			this.listBox1.Name = "listBox1";
			this.listBox1.Size = new System.Drawing.Size(248, 95);
			this.listBox1.TabIndex = 2;
			// 
			// textBox2
			// 
			this.textBox2.Location = new System.Drawing.Point(120, 40);
			this.textBox2.Name = "textBox2";
			this.textBox2.TabIndex = 3;
			this.textBox2.Text = "textBox2";
			// 
			// textBox3
			// 
			this.textBox3.Location = new System.Drawing.Point(8, 64);
			this.textBox3.Name = "textBox3";
			this.textBox3.TabIndex = 4;
			this.textBox3.Text = "textBox3";
			// 
			// textBox4
			// 
			this.textBox4.Location = new System.Drawing.Point(120, 64);
			this.textBox4.Name = "textBox4";
			this.textBox4.TabIndex = 5;
			this.textBox4.Text = "textBox4";
			// 
			// textBox5
			// 
			this.textBox5.Location = new System.Drawing.Point(8, 88);
			this.textBox5.Name = "textBox5";
			this.textBox5.TabIndex = 6;
			this.textBox5.Text = "textBox5";
			// 
			// textBox6
			// 
			this.textBox6.Location = new System.Drawing.Point(120, 88);
			this.textBox6.Name = "textBox6";
			this.textBox6.TabIndex = 7;
			this.textBox6.Text = "textBox6";
			// 
			// Form2
			// 
			this.AutoScaleBaseSize = new System.Drawing.Size(5, 13);
			this.ClientSize = new System.Drawing.Size(292, 266);
			this.Controls.Add(this.textBox6);
			this.Controls.Add(this.textBox5);
			this.Controls.Add(this.textBox4);
			this.Controls.Add(this.textBox3);
			this.Controls.Add(this.textBox2);
			this.Controls.Add(this.listBox1);
			this.Controls.Add(this.textBox1);
			this.Controls.Add(this.label1);
			this.Name = "Form2";
			this.Text = "Form2";
			this.ResumeLayout(false);

		}
		#endregion
	}
}

run the form and go to file-> testForm, see how it flickers when the controls are created, now move the child form around, see how the controls are not moving smootly.

remove the line:
this.Opacity = 0;

and try again, this fixes the problem, go figure....

I'll never play with this opacity thigny again.

HTH.