You can use Font Finagler's Font Cache Cleaner feature to purge cache files in an attempt to restore normal behavior, and to help rule out a font cache corruption as the sole cause of the problem.

If, however, you use Font Cache Cleaner but the problem remains, then it could be caused by a font conflict, by a corrupt or less-than-ideal-quality font, or by an underlying OS X font-handling bug.

You can try removing fonts to isolate the problematic fonts that are causing this behavior.