This happened to me recently. I solved it by deleting all the Cache files and cleaning the Database.
Thanks for that. New to Premiere so not sure which files to wipe.
Guessing folder is Library\Preferences\AdobePremiere Pro\6.0 but not
sure from there.
Edit/Preferences/Media and then select Clean Media Cache. After loading a project wait for the indexing, conforming and creating peak files to finish before starting editing.
The Clean button referenced above only works on the Media Cache Database, and is part of the process. However, you'll have to manually delete the actual Media Cache files from Explorer or Finder. You can see their hard drive location at the top of the same Preference window.