I had the same problem some time ago, something was using all the memory available and that was the reason. The logs didn't say anything too.
If you can install software on your server I recommend you to install webmin, and then configure one monitor on Apache to issue a restart command when it detects that the server is down.
Other -> System and Server Status -> Click on Apache Webserver ->
If monitor goes down, run command -> apachectl -k restart
This is doing the job for me...
roger
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