Every 2 days Error 500 on shop

-- HikaShop version -- : 2.6.1
-- Joomla version -- : Joomla! 3.4.4
-- PHP version -- : PHP Version 5.5.9-1ubuntu4.14
-- Browser(s) name and version -- : ALL
-- Error-message(debug-mod must be tuned on) -- : Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Please contact the server administrator at This email address is being protected from spambots. You need JavaScript enabled to view it. to inform them of the time this error occurred, and the actions you performed just before this error.

More information about this error may be available in the server error log.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Hello,
I have installed latest version of Hikashop on Joomla 3.4
Sometimes, every 1 or 2 days I have error 500 on the shop.

This is my PHP info:

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On the error log of the server I have several:

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I need to ask my hosting administrator to reboot the machine every time. Please help me, I can modify php.ini with your suggestion.

Hi,
here it is:
[Wed Feb 10 00:26:17.476384 2016] [fastcgi:error] [pid 58312] (104)Connection reset by peer: [client *.*.*.*:18050] FastCGI: comm with server "/var/www/***" aborted: read failed
[Wed Feb 10 00:26:17.476446 2016] [fastcgi:error] [pid 58312] [client *.*.*.*:18050] FastCGI: incomplete headers (0 bytes) received from server "/var/www/***"

We still continue having many errors.

[Tue Feb 23 16:53:31.356943 2016] [fastcgi:error] [pid 14016] [client 79.1.123.71:51791] FastCGI: incomplete headers (0 bytes) received from server "/var/www/clients/client2/web60/cgi-bin/php5-fcgi-185.109.184.140-80-colossiarte.it", referer: www.colossiarte.it/administrator/index.php?option=com_hikashop
[Tue Feb 23 16:53:31.934758 2016] [fastcgi:error] [pid 14016] (104)Connection reset by peer: [client 79.1.123.71:51791] FastCGI: comm with server "/var/www/clients/client2/web60/cgi-bin/php5-fcgi-185.109.184.140-80-colossiarte.it" aborted: read failed, referer: www.colossiarte.it/administrator/index.php?option=com_hikashop
[Tue Feb 23 16:53:31.934857 2016] [fastcgi:error] [pid 14016] [client 79.1.123.71:51791] FastCGI: incomplete headers (0 bytes) received from server "/var/www/clients/client2/web60/cgi-bin/php5-fcgi-185.109.184.140-80-colossiarte.it", referer: www.colossiarte.it/administrator/index.php?option=com_hikashop

Where we can read PHP error log?