blank page

I am using 2.6.2 and have been having on and off problems when posting or updating a post. I can’t find a consistent pattern. Sometimes it works, sometimes I get either a blank page or the website loads with the text “Sorry, but you are looking for something that isn’t here.” This happens on pages that do exist like edit.php and post.php I am having these problems in the wp-admin part, not viewing the website. I tried different web browsers, windows,linux. Same thing, most the times it works, but then sometimes it wont. Any suggestions?