If your sites all of a sudden show a 500 internal server error and your Apache logs show "Premature end of script headers" then you may want to check your logs to be sure they are not too large. Usually the logs would be under your apache directory (/usr/local/apache or /etc/httpd). This can also happen if a file that is called by the script is above the 1G mark. This will cause these errors as well. To find the file, go to the directory of the script/account and run the following as root 'find ./ -size +1G' that should tell you what files are over the 1G size.
If it's not that, it's possible that your /tmp directory may be mounted with noexec and is causing some of your scripts to not function correclty.
Lastly, if you are running suphp, make sure none of your folders are 777.
To re-enable exec for /tmp, run the following command:
'mount -o remount,exec /tmp'