What can cause all services to CRASH?

I have this problem where every few days my server crashes and brings down all the services apache ftp even SSH, what could cause something like this?

My server provider has done extensive hardware tests, and they ruled it out as being a hardware issue, could it be due to software issues?

I'm running Centos 3.3 with cpanel. Could someone pls help?

 

 

 

 

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