request failed: error reading the headers

[cmac]

Recently I moved my servers from one provider to the other and started to get this messages in apache error log: "request failed: error reading the headers"

Example from the error and corresponding access apache logs:
ApacheServer$ cat error_log
ApacheServer [Tue Jan 20 11:07:44 2015] [error] [client x.x.x.x] request failed: error reading the headers

ApacheServer$ cat access_log
x.x.x.x - - [20/Jan/2015:11:06:44 +0200] "GET SomeRandomRequest HTTP/1.1" 400 226 "-" "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.3; WOW64; Trident/7.0; .NET4.0E; .NET4.0C; .NET CLR 3.5.30729; .NET CLR 2.0.50727; .NET CLR 3.0.30729; BRI/2; MAARJS)" "-"

I must admit that nothing is changed besides php version (from 5.3 to 5.4).
I don't see any relation to some specific browser or some specific request, it is totally random.
Also it can not be the switch problem because I have the same problem on my other apache servers that are not sharing same switch.

Till now I tried to:
- update network card driver
- raise RX from 256 to 4096 with: ethtool -G eth1 rx 4096
- bond second network card to the interface
- google this problem many times but didn't find the solution or same issue.

Server version: Apache/2.2.15-39 (Unix)
centos 6.5