I have checked your code there is no problem which you have wrote.. I have only remark the line of varMode because I have not understand why you have use this..
Private Sub DataGridView1_CellEnter(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView1.CellEnter
If DataGridView1.Rows(e.RowIndex).Cells(Me.ComboColumn6.Name).Selected = True Then
DirectCast(DataGridView1.EditingControl, DataGridViewComboBoxEditingControl).DroppedDown = True