ComboBox problem

I have MyComboBox owner-draw control.
In OnDropDown I do SetCurSel(-1), because I want to scroll my window by myself. Then I post my user message.
I select an item in my user message (works OK), but after UserMessage has finished, item is unselected!!!! How could I select an item in UserMessage function?
I can't use SetCurSel or SetTopIndex because they autoscroll window.
Thanks all!

void CMyComboBox::OnDropdown()
{
   ...
   SetCurSel(-1);
   PostMessage(WM_USERMESSAGE);
}

void CODComboBoxDB::OnUserMessage(...)
{
   m_MyListBox.SetCurSel(x)
}