Cannot switch tty's after install xorg, gnome and gdm

Hi guys.
On every release of freebsd that i tried (-CURRENT, -STABLE, -RELEASE)
everything goes fine until I install xorg, gdm and gnome by packages
or not it does not make any difference. When I boot with
gnome_enable="YES" , and gdm start, when I try to change tty, on every
tty, its a black screen and my monitor tell me, power saving mode
enable until I switch back to gdm. I installed the xf86-video-ati
driver and I have an ati radeon HD 5770.