Both my gentoo box and my fedora 20 box are doing the same thing. My gentoo box is running 331.38 drivers with 1.15 xorg server. If I switch to tty2 by ctrl alt f2 and go back to my desktop on tty7, the screen is garbled and I have to zap the x server and start over, is anyone else noticing this? Other than that everything else appears normal and there is nothing relevant in the xorg log
If you use kde with compositing opengl effects, then i noticed it too in this and previous driver release.
You can workaround by disabling and enabling the effects by a ctrl-alt-f12 twice.
Actually, I found the problem thanks to your post, under desktop effects advanced settings, change open gl from 2.0 to 3.1