HELP! System keeps locking up

Hello, all... I'm running Fedora 20 on a homebrew PC It's an AMD Athlon X2 processor with an on-board nVidia display chipset. This is a fresh install of F20, pretty much up to date. I had to do a fresh install since I mangled the upgrade from F18. Every once in awhile, mainly when I'm trying to unlock the screen, the system will freeze up. My display is a black background with "bits" of color in a diagonal pattern. I say "mainly when trying to unlock the screen" because I had it happen to me this morning (third time in 24 hours) when I was thinking of trying a different screen saver (been using Moebius Gears) on the thought that maybe the Noveau driver doesn't like the GL screen saver for some reason. Every time this happens I have to use the reset button on my system. This morning when it happened, I had an SSH window from another PC logged in and it was frozen as well. I have checked /var/log/messages and there is nothing in there that would indicate a problem. There was hardly anything in there since about 8PM last night and it's locked up twice since then. Suggestions/thoughts/theories??? I'm running XFCE, if it matters, but still using parts of KDE as well.