在'读取初始通信包'时失去与MySQL服务器的连接,系统错误111

I know there are quite a few questions with the same error here, but none of them refers to the same scenario as mine so please - do not mark it as duplicate.

I have a dedicated server with HeartInternet and I host around 50 websites on it. For connection I use PHP PDO and the NON persistent connection.

A few times a day all of the websites are down displaying the following error:

SQLSTATE[HY000] [2013] Lost connection to MySQL server at 'reading initial communication packet', system error: 111

HeartInternet says that it's nothing to do with them and I know the code for the database wrapper is written properly. Any idea what might be causing it?

Could the fact that there are 50 sites hosted on one machine have something to do with it? Should we use MySQL cluster for this sort of volume?