combobox keypress event problem

Use the key down event.

private void comboBox1_KeyDown(object sender, KeyEventArgs e)
 {
     if (e.KeyValue == 13)
     {
         textBox1.Focus();
     }
}

In VB

Private Sub combobox1_KeyDown(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles ComboBox1.KeyDown
      If e.keyValue=13 Then
         textBox1.Focus();
      End If
End Sub

Take a look I've again checked your ComboBox1_KeyPress Event Coding as given
as You mentioned in Comment you are adding Some value in ComboBox in Form_Load event. I've also write a function which will able to fill ComboBox in FormLoad event

Private Sub ADDData(ByVal cmb As ComboBox)
      Dim i As Integer
      For i = 0 To 10
          cmb.Items.Add(i) ' Add value from 1 to 10 in corresponding combobox 
      Next
  End Sub

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
       ADDData(ComboBox1) 'Calling method in form_load event for adding value im Combobox 
   End Sub

Private Sub ComboBox1_KeyPress(ByVal sender As System.Object, ByVal e As System.Windows.Forms.KeyPressEventArgs) Handles ComboBox1.KeyPress
         If Asc(e.KeyChar) = 13 Then
           e.Handled = True
           TextBox1.Focus()  'it focus on TextBox1 after selecting value and press enter 
           Exit Sub
       End If
   End Sub

This is not a solution but more of a confirmation of the issue.

For me the focus goes completely out of the app. For me this is manifesting itself with only one app and occurs only once after I start the app and only the first time I use the combo box. Then intermittently but not consistent enough to draw any further deductions.

I have not yet found a solution to this but I can clarify how to duplicate.

The app is a basic DB list on the main screen using a C1 True Grid. A toolbar button is clicked where I spin up an Entry Form instance (The Entry Form is where the issue manifests itself) and do a show dialog. (The Entry Form is a simple. Not much in the form of business logic.)

I select an entry in the combobox with my mouse and press Enter. The Windows focus goes out in space. The only way to regain App focus is to click on the entry Form with my mouse.

I will simply explain it to my users and live with it. BTW...If I tab through the control order the problem does not occur.