White blank page when Updating/Draft/Publishing Page/Post

On my VPS (centos 6.7 64x, 2 cpu's and 3gb ram) I have setup a WP MU(Memory usage: 53.97 MByte)and many times I have a blank page when I make a post draft or publish/updating it, or when I want to move as an admin to another option in wp.

I have added on the top of the file wp-config.php

define('WP_MEMORY_LIMIT', '256M');
define('WP_MAX_MEMORY_LIMIT', '512M');

Also, by doing 'inspect element on chrome, I noticed that it shows me this this error:

Failed to load resource: net::ERR_CONTENT_LENGTH_MISMATCH

I enable the debugging

// Turn debugging on
define('WP_DEBUG', true);

// Tell WordPress to log everything to /wp-content/debug.log
define('WP_DEBUG_LOG', true);

// Turn off the display of error messages on your site
define('WP_DEBUG_DISPLAY', false);

// For good measure, you can also add the follow code, which will hide errors from being displayed on-screen
@ini_set('display_errors', 0);

I decided to make a new post and save it as a draft. It showed me a blank white page while it was saving it. And I got this on debug.log

[22-Nov-2015 21:37:02 UTC] 
--------------------------------------
REQUEST: /wp-admin/post.php


[22-Nov-2015 21:37:04 UTC] 
--------------------------------------
REQUEST: /wp-admin/admin-ajax.php


[22-Nov-2015 21:37:04 UTC] 
--------------------------------------
REQUEST: /wp-admin/post.php?post=34890action=editmessage=10


[22-Nov-2015 21:37:10 UTC] 
--------------------------------------
REQUEST: /wp-admin/admin-ajax.php


[22-Nov-2015 21:39:04 UTC] 
--------------------------------------
REQUEST: /wp-admin/admin-ajax.php

I never had this problem before (I use the same theme/plugins) I upgrade (centos from 5.5 to 6.7 64bit)the os of the server(I don't if this has something to do with the problem). I use the same theme/plugins.

Update I increased apache/php limits by adding the following 2 lines in /etc/httpd/apache/conf/php.conf

FcgidIOTimeout 400

MaxRequestLen 20468982

# This file was automatically generated by the Cpanel PHP Configuration system
# If you wish to change the way PHP is being handled by Apache on your system,
# use the /usr/local/cpanel/bin/rebuild_phpconf script or the WHM interface.
#
# Manual edits of this file will be lost when Apache is updated.


# Fastcgi configuration for PHP5
LoadModule fcgid_module modules/mod_fcgid.so
MaxRequestsPerProcess 500
AddHandler fcgid-script .php5 .php4 .php .php3 .php2 .phtml
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .php5
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .php4
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .php
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .php3
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .php2
FcgidWrapper /usr/local/cpanel/cgi-sys/php5 .phtml

# End of autogenerated PHP configuration.
FcgidIOTimeout 400
MaxRequestLen 20468982

Before, every time I were publishing/update or draft a post it was showing the problem on every thing. Now, the problem is random since I increased the apache/php limits. Does it mean that something else (how can I find it?) is putting load on vps or do I have to increase more the apache/php?

I have added w3tc plugin and optimised the sites but it didn't change anything. Should I add apcu or memcache / memcached combined with varnish or more RAM?

P.S.:This is my.cnf in the case that is needed as extra info to fix the problem that I have.