[snakes1100]

As its only a "sometimes", its not going to be easy to track down, typically this is cause by a redirect issue in htaccess by url rewrites.

You're going to have to inspect your http logs when it happens or know a time when it happens to see what may be in the logs, a lot of times, you will need debug on in http to even see a error of this type.

When it does happen, rename your .htaccess file to htaccess & see if the error goes away as a quick test.