This can be related to one of two issues, sometimes both.
1. WordPress likes to send pingbacks to all of the links that you have included with your post. When it does this, it kindly waits for a response from the pinged host before proceeding. If it does not receive an immediate response, it will wait. Technically, this is not a problem with WordPress, but rather a problem with the remote host’s ability to quickly return a simple ping. Also, in some cases, the remote host may be down and completely unable to return the ping. In this case, WordPress will eventually give up and return you to your posting page, or your server may give up first and return a momentary “server stopped responding” error.
2. WordPress also likes to ping whichever update services that you have listed in Options/Writing. Once again, WordPress will wait for a response before proceeding. Technically, this is also not a problem with WordPress, but rather a problem with the remote host’s ability to quickly return a simple ping. Also, in some cases, the remote host may be down and completely unable to return the ping. In this case, WordPress will eventually give up and return you to your posting page, or your server may give up first and return a momentary “server stopped responding” error. As a side note, the pinging mechanism of Pingomatic (a popular update service) has been having some serious issues as of late, which are currently affecting its availability.