Ever since I started using OS X, I have noticed strange rendering artefacts when viewing PDFs I create using pdfTeX, XeTeX, or LuaTeX using Preview.app. These artifacts are not always reproducible, but they do appear frequently. Interestingly, I have not observed these artefacts when viewing PDFs that I download from other sources. When I select the text (e.g. using Command-A or my cursor), these artefacts disappear. Do you have any idea what might be causing them, and what I can do in order to fix the issue?
I have attached an image that contains several of these rendering artefacts below.