Can't boot Live USB into Ubuntu 16.04: Flashing Icons, or nothing at all

I'm trying to diagnose problems with my new 16.04 install, so I tried to boot into the LIVE USB for 16.04. I see this error message during the process

nouveau: failed to load fecs_inst

Also, when it finally boots, I only see a screen with the pink/purple ubuntu background. No icons or panels. If I right-click I might get a menu where I can create a new folder, or launch a terminal, but then that menu and those icons are flashing at a rate of about once per second.

Does anyone know how I can fix this? I've already spent 10+ hours trying to install first Ubuntu 15.10, and now 16.04.