Upgrade.php on line 18 issue

The issue I am having is that every now and then, when I am in the admin section of the site, doing admin stuff like deleting comments, I get the following error:

Warning: require(../wp-load.php) [function.require]: failed to open stream: No such file or directory in /home/nemo-maritime/public_html/wp-admin/upgrade.php on line 18

Fatal error: require() [function.require]: Failed opening required '../wp-load.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/nemo-maritime/public_html/wp-admin/upgrade.php on line 18

I am assuming that the issue is because of upgrade.php. The issue comes and goes, and when it happens, I just wait a bit, and then go back to mysite.com/wp-admin and I am back in the admin section. Then a little while later, the issue comes back.