You can post in other subforums though, it seems? And you sometimes can post in this forum and sometimes you can't?
Have you tried with another browser like http://www.google.com/chrome
Do you have any firewall or web filtering software running on your computer? You could try disabling it temporarily just to see if that has any effect.
Next time you try to post and it fails, if you can note the exact time+date of the failure, reply with it here or PM me and I might be able to help more.