Error: Warning: include_once(x) [function.include-once]: failed to open stream: No such file or directory..

This error occurs RANDOM, sometimes it's accessable, sometimes it's not.
I'm running my own webserver and mysql database, I'm the only one using the server and the webserver, and the only one programming this particular project.

I'm wondering what could be causing this issue, since the file DO EXIST and it's accessable most of the time, but sometime it just gives me this error, this is fixed after about 15 seconds (automaticly) or if I restart the webserver.

Solution?