I suspect the lockup during the graphical session is due to an nvidia problem.

I'd like to downgrade to kmod-nvidia-173xx-2.6.40.3-0.fc15.i686-173.14.31-1.fc15.1, but there are several conflicts with various xorg files that I am not sure how to handle.  

Has anyone successfully downgraded kmod-nvidia and solved this problem ?

Thanks