Probably your Web server OOMing the box? Check dmesg.

Apache-specific advice: if you're on a small VPS or other box with not much RAM, and didn't tweak the distribution defaults for MaxClients and friends in its configuration, a HN-size load will OOM you rapidly, especially with mod_php. The defaults for prefork MPM are nuts in the major distros (as in, assuming 4 GB of RAM or more for mod_php).

Since you're going up and down, I assume this is the case. I suggest dropping MaxClients to 5 for now and letting us wait a bit.