Forbidden and no access

I’m on WordPress 2.3.3 and I’m using a hosting company.

Occasionally I get this problem.

I’m trying to save a post and halfway through, I get this message:

Forbidden: You do not have permission to access this document.
Web Server at xxxx

Once I get this message, I cannot save anything else on the post (except whatever else that has been saved earlier).

Even if I create a new post and start re-writing the same article, I will soon get another Forbidden message.

The Forbidden message also appears if I deleted the existing post and start a new one with the same content.

However, there’s no problem if I start writing a different subject on a new post.

I wonder if anyone has experienced a similar problem and, more importantly, how to sort this out.

Thank you.