screen goes black in fedora 20 live cd

[Fatemeh Mehdizadeh]

Hi all,
I want to use live cd of fedora 20 but when it boots up suddenly it's
screen goes
black.
I search about this and do the following steps:
1. In /etc/default/grub, I remove "nomodeset"
2. grub2-mkconfig -o /boot/grub2/grub.cfg
3.reboot

Now my system is very bright and nice but just for some minutes! After some
minutes the screen goes black. when I add nomodeset I don't see black
screen but in live cd I see it. Also with nomodeset it can't recognize my
graphic card(radeon).

Thanks.