Online flash videos such as youtube videos don't play well on Ubuntu 9.04
The default flash plugin for Ubuntu 9.04 is set to swfdec instead of adobe flash. swfdec seems to be buggy at the moment.
- Uninstall following packages : mozilla-swfdec, libswfdec
sudo apt-get remove mozilla-swfdec libswfdec
- Install Adobe flash plugin 10 from its site.
This should fix the problem.