Intermittent PHP memory exhausted when using W3TC

Hi everyone,
I use W3 total cache on my website. At first sight, it seems like everything works just fine : the loading speed of my home page has increased. However, it comes at a price, since I sometimes get an error 500 due to a PHP memory exhausted.
Of course, I have tried to set the PHP memory limit higher and higher, to indecent levels, but this does not solve the problem. The error disappears when I deactivate the plugin W3TC.
Here is the version of W3TC I use : w3-total-cache.0.9.5.2.
Here is my php info :
ubuntu@skilero:~$ php -i
phpinfo()
PHP Version => 7.0.15-0ubuntu0.16.04.2

System => Linux skilero 4.4.0-64-generic #85-Ubuntu SMP Mon Feb 20 11:50:30 UTC 2017 x86_64
Server API => Command Line Interface
Virtual Directory Support => disabled
Configuration File (php.ini) Path => /etc/php/7.0/cli
Loaded Configuration File => /etc/php/7.0/cli/php.ini
Scan this dir for additional .ini files => /etc/php/7.0/cli/conf.d
Additional .ini files parsed => /etc/php/7.0/cli/conf.d/10-mysqlnd.ini,
/etc/php/7.0/cli/conf.d/10-opcache.ini,
/etc/php/7.0/cli/conf.d/10-pdo.ini,
/etc/php/7.0/cli/conf.d/15-xml.ini,
/etc/php/7.0/cli/conf.d/20-apcu.ini,
/etc/php/7.0/cli/conf.d/20-calendar.ini,
/etc/php/7.0/cli/conf.d/20-ctype.ini,
/etc/php/7.0/cli/conf.d/20-dom.ini,
/etc/php/7.0/cli/conf.d/20-exif.ini,
/etc/php/7.0/cli/conf.d/20-fileinfo.ini,
/etc/php/7.0/cli/conf.d/20-ftp.ini,
/etc/php/7.0/cli/conf.d/20-gettext.ini,
/etc/php/7.0/cli/conf.d/20-iconv.ini,
/etc/php/7.0/cli/conf.d/20-json.ini,
/etc/php/7.0/cli/conf.d/20-mysqli.ini,
/etc/php/7.0/cli/conf.d/20-pdo_mysql.ini,
/etc/php/7.0/cli/conf.d/20-phar.ini,
/etc/php/7.0/cli/conf.d/20-posix.ini,
/etc/php/7.0/cli/conf.d/20-readline.ini,
/etc/php/7.0/cli/conf.d/20-shmop.ini,
/etc/php/7.0/cli/conf.d/20-simplexml.ini,
/etc/php/7.0/cli/conf.d/20-sockets.ini,
/etc/php/7.0/cli/conf.d/20-sysvmsg.ini,
/etc/php/7.0/cli/conf.d/20-sysvsem.ini,
/etc/php/7.0/cli/conf.d/20-sysvshm.ini,
/etc/php/7.0/cli/conf.d/20-tokenizer.ini,
/etc/php/7.0/cli/conf.d/20-wddx.ini,
/etc/php/7.0/cli/conf.d/20-xmlreader.ini,
/etc/php/7.0/cli/conf.d/20-xmlwriter.ini,
/etc/php/7.0/cli/conf.d/20-xsl.ini

Thank you,

Amo