I don't know if this is caused by Xorg or by KDE, but it doesn't happen in xfce.

In all of my machines with various video cards, and in both 14.0 and 14.1, KDE has the following 2 problems:

(1) All the tooltips that appear when hovering over things in a toolbar are surrounded by a humongous black rectangle. The black rectangle is about 4 times the size of the actual tooltip. The same thing happens when you hover over things in the panel at the bottom of the screen.

(2) Things that should be highlighted because of being selected, such as items in a directory list, get turned into a multicolored band that completely washes out the text that is being highlighted.

I have brought forward my own modelines that I've been using for 10 or 15 years in order to always have acceptable modes. All my systems are fresh installs from the cd's.