Artifacts when viewing pdfs in Preview (and Skim)
The first, simplest step for troubleshooting even an Apple app that is acting up is to quit it and delete its preference files. There are some utilities that check the validity of preferences files and delete corrupt ones. Easier still is to move them to the trash from your 
User Library at ~/User/Library/Preferences 
or the Main Library at HardDisk/Library/Preferences. The application will rebuild a fresh set next time you open it. 

(Note I am not referring to the Library within the System Folder.)

If there are complicated settings you might want to remember, make a sticky first!

The second step is often to download or otherwise install a fresh copy of the application itself. It's surprising what both these steps can accomplish.