Custom combobox control ignores first tab key press

Question

I have a very simple custom control based on the combobox control, that just adds the ability to provide a prompt for the user and adjust the color of the prompt text. It works well, except that if I have several controls on a form, when I enter one of my custom cb controls and then try to tab on to the next control, I always have to hit tab twice before it moves to the next control. I have no idea why it is 'swallowing' the first tab key press, then works fine with the second one.

I have been researching this on StackOverflow and elsewhere for awhile now, but can't find anything addressing this issue - only advice on how to capture the tab key press by overriding the 'OnPreviewKeyDown' event, but I don't really need to do anything different with the tab key than it's already doing, I just need it to work on the first key press.

Here is the custom control code:

    public partial class PromptComboBox : ComboBox
    {
        string prompt;
        Color promptColor;
        Color savedColor;

        [Description("Prompt displayed to user"), Category("Appearance"), DisplayName("Prompt Text")]
        public string Prompt
        {
            get { return prompt; }
            set
            {
                prompt = value;
                Text = prompt;
            }
        }

        [Description("Color for Prompt Text"), Category("Appearance"), DisplayName("Prompt Color")]
        public Color PromptColor
        {
            get { return promptColor; }
            set
            {
                promptColor = value;
                ForeColor = promptColor;
            }
        }

        public PromptComboBox()
        {
            savedColor = ForeColor;
            ForeColor = promptColor;
            InitializeComponent();
        }

        protected override void OnSelectedIndexChanged(EventArgs e)
        {
            if (SelectedIndex == 0)
            {
                ForeColor = promptColor;
                Text = prompt;
            }
            else
            {
                ForeColor = savedColor;
            }

            base.OnSelectedIndexChanged(e);
        }

        protected override void OnKeyPress(KeyPressEventArgs e)
        {
            if(char.IsLetterOrDigit(e.KeyChar) || char.IsPunctuation(e.KeyChar))
            {
                if (Text == prompt) Text = "";
            }

            base.OnKeyPress(e);
        }

        protected override void onke

        protected override void OnKeyUp(KeyEventArgs e)
        {
            if (Text.Length == 0 || (Text == prompt && (e.KeyCode == Keys.Back)))
            {
                ForeColor = promptColor;
                Text = prompt;
            }            
            else
            {
                ForeColor = savedColor;
            }

            base.OnKeyUp(e);
        }

        protected override void OnEnter(EventArgs e)
        {
            //base.OnEnter(e);
            if (Text == prompt) { Select(0, 0); }
        }

        protected override void OnLeave(EventArgs e)
        {
            if (Text.Length == 0 || Text == prompt)
            {
                ForeColor = promptColor;
                Text = prompt;
                Select(0, 0);
            }

            base.OnLeave(e);
        }

        protected override void OnClick(EventArgs e)
        {
            if (Text == prompt) { Select(0, 0); }
            base.OnClick(e);
        }

    }

Any help or advice would be much appreciated!