EDIT: MY problem turned out to be corrupt files on my unraid server due to a bad RAM chip on the server.
I have ~100 movies in x264 mkv with file sizes between 8-15GB. All of them display the grayish pixelation. They'll be ok for 5-15 minutes and then as scenes change they'll receive the pixelation. Sometimes it lasts for only a second or two, other times it'll compound until the screen is covered with it. Once a scene changes it'll often clear the gray out.
I've tried Windows Media Player 12, VLC, and xbmc. I've also tried Windows 8.1 and ubuntu 12 (vlc, xbmc). I've tried two machines. I've tried playing them off of my file server, as well as copying them locally.
At the least, does anyone know what this type of pixelation is called? I've spent hours trying to find out, but it's hard to search for.
Machine1:
Windows 8.1
i7 930 @ 2.8GHz
GTX 480 SLI (x2)
12 GB ram
Machine 2:
Win8 and also tried ubuntu 12.10
i3 4130T haswell 2.9GHz
integrated intel 4400 graphics
4 GB ram