Why would you need that? If there is a memory leak or something, that needs to be addressed first. Solve the problem, not the symptom.
But that wasn't your question was it? I understand that is probably what you've already tried.
I would experiement with putting it into a PHP script that simply does a `reboot`; I am not sure if you could launch reboot directly from a cron job.
*Notice the backwards quote.
Then your cron job does:
php /home/folder/reboot.php