I have this problem too on two servers:
home: MODx Revolution 2.0.0-pl, tinymce: 4.0.4 rc2, Apache: 2.0, PHP Version: 5.3.3, Mysql: 5.1.47
production: MODx Revolution 2.0.0-pl, tinymce: 4.0.4 rc2, Apache: 2.0, PHP Version: 5.3.3, Mysql: 5.0.7-dev
Browsers: latest Firefox and Chrome
Saving some modDocument resources hangs with "Please Wait / Saving..." message: