Recent yum updates to my omen.com server have resulted in an unstable system. At first the problem appeared to be related to the Tigervnc server. VNC sessions would fail quickly. I ended up force installing Tigervnc from the Heisenbug tree.
With VNC apparently settled down, I saw hard disk errors on various drives. The solution for that was to fall back to 3.16.0-0.rc4.git2.1.fc21.x86_64. Omen.com now seems stable.
Meanwhile Fedora 21 and rawhide installs on my test machines ceased to boot into a graphical desktop. I tried both Gnome and Xfce. I could get a text console but no desktop. The current F21 anaconda didn't get to a desktop screen either.
I then replaced the Nvidia FTX 460 on my spare test machine and F21 anaconda started working.
Since my "ultimate goal" is to get a (relatively) clean F21 on my server, I started experimenting using the Heisenbug netinst.iso with a specified repository of http://mirrors.kernel.org/fedora/development/21/x86_64/os
It would be most useful to be able to specify a copy of the repository on a local HD: /dev/sdd1/fc21/os - but in the meantime I will have to install over the net at a more leisurely pace.