I ended up solving this by reading this article http://www.rqna.net/qna/znvkk-wpf-richtextbox-selected-text-distorted.html
This line of code in the xaml.cs constructor seems to clear it up.
RenderOptions.ProcessRenderMode = System.Windows.Interop.RenderMode.SoftwareOnly;
It is still slightly blurry but at least readable now.