Thanks,
Yep, looks like php and lighttpd might have a memory leak, or, cache size too big.
Any other logs would be useful?
cat /var/log/lighttpd/error.log
cat /var/log/php5-fpm.logp
And lets check cache sizes, should be around 50/60MB each:
http://IP/apc.php
http://IP/opcache.php