Dialog disappers when DropDown is shown (on top of AcroPDF viewer)

I have a Winform modal form with our home-made ComboBox on it, which functions well usually. My problem is - when this modal form is opened on top of a main form that has an AcroPDF control on it (Adobe's PDF ActiveX), whenever the combo's drop down is displayed, the entire modal form disappears. Clicking the form's area brings the form back to view. Another problem (probably similar) - when hovering a contorl that displays tooltip - whenever the tooltip is displayed, the same thing happens - the modal form disappears

Does anyone have an idea?

Thanks