wpf in vista (transparent window problem)

Hi,
I wrote a WPF Aplication that have a transparent window with tree buttons
that a timer event move them over the transparent window, in window XP thay
move fast and smootly but in vista the hang and move very slow!!!

Can you tell me why???

I try that in 3 different computer with different screen card and i get the
same problem only in vista!!!

Thanks.