Intermitently not able to edit type in an existing photoshop document. Closing the application and restarting temporarily fixes the issue but then after about 30mins the same problem continues. It applies to any fonts placed in the document. Not just one.
Are Photoshop and OS fully updated and have you performed the usual trouble-shooting routines (trashing prefs by keeping command-alt-shift/ctrl-alt-shift pressed while starting Photoshop, 3rd party plug-ins deactivation, system maintenance, cleaning caches, font validation, etc.)?
Are you still experiencing the problem after trying the steps Christopher mentioned? If so, could you send me one of the files you're having trouble with, so I can determine if it's a file or application problem? I will send you a private email to follow up.