From memory, just after f15 was released, i had installed ALL available DE's on it.
As you can imagine the system went unstable due to diffrent dependencies and conflicts of them to each other.
Before doing anything further, i'd suggest you do backup the most important data.
Then, while beeing loged in shell as root, remove the dirs /home/Onerous/.config
Further some of the ~/.X... files too as they store the X11 settings / values which might cause errors for the one or the other.
Deinstalling a DE would be another option, and is quite easy to be accomplished..
Just type either one of these lines as root:
yum groupremove "GNOME Desktop Environment"
yum groupremove Xfce
These options are pretty radical, you might want to try other inputs you hopefully get here.
Hope this helps