Try to use SetWindowPos WinAPI function to make a Delphi-form to stay on top: And for lose stay on top, you should change HWND_BOTTOM to HWND_TOPMOST in this function.
SetWindowPos( Handle, HWND_TOPMOST, 0, 0, 0, 0, SWP_NOACTIVATE or SWP_NOMOVE or SWP_NOSIZE);