High disk I/O rate - what's happening?

I'm running a Linode instance with a WordPress blog. The config is simple: Apache2 working together with Ningx acting as a reverse proxy. Everything seemed to be fine, but from time to time the server is not responding and I get a Linode alert with the notification about a high I/O disk rate.

I've checked memory (free -m) and everything is OK right now, but I can also show the last apache2 errors:

[Thu Feb 17 02:07:25 2011] [notice] child pid 14271 exit signal Segmentation fault (11) [Thu Feb 17 02:07:27 2011] [notice] child pid 15099 exit signal Segmentation fault (11) [Thu Feb 17 02:07:27 2011] [notice] child pid 15334 exit signal Segmentation fault (11) [Thu Feb 17 02:07:29 2011] [notice] child pid 13316 exit signal Segmentation fault (11) [Thu Feb 17 11:21:33 2011] [notice] child pid 14286 exit signal Segmentation fault (11) [Fri Feb 18 06:36:51 2011] [error] [client 193.164.131.205] File does not exist: /var/www/webdav [Fri Feb 18 06:56:30 2011] [error] [client 66.176.46.168] File does not exist: /var/www/webdav [Fri Feb 18 07:49:25 2011] [error] [client 174.91.232.124] File does not exist: /var/www/webdav [Fri Feb 18 09:18:03 2011] [notice] Apache/2.2.14 (Ubuntu) PHP/5.3.2-1ubuntu4.7 with Suhosin-Patch configured \ -- resuming normal operations [Fri Feb 18 09:19:03 2011] [notice] caught SIGTERM, shutting down [Fri Feb 18 09:19:23 2011] [notice] Apache/2.2.14 (Ubuntu) PHP/5.3.2-1ubuntu4.7 with Suhosin-Patch configured \ -- resuming normal operations [Fri Feb 18 11:04:10 2011] [notice] child pid 2666 exit signal Segmentation fault (11) [Fri Feb 18 11:04:10 2011] [notice] child pid 2670 exit signal Segmentation fault (11) [Fri Feb 18 11:04:11 2011] [notice] child pid 2643 exit signal Segmentation fault (11) [Fri Feb 18 12:11:21 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:15:09 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:18:59 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:20:03 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:24:52 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:24:52 2011] [error] [client 81.35.19.228] File does not exist: /var/www/feed [Fri Feb 18 12:26:53 2011] [error] [client 127.0.0.1] request failed: error reading the headers [Fri Feb 18 12:28:40 2011] [notice] Apache/2.2.14 (Ubuntu) PHP/5.3.2-1ubuntu4.7 with Suhosin-Patch configured \ -- resuming normal operations

Don't know anything about the webdav errors (do I have to configure that? never heard about it) and I'm not sure if the problem is related with Suhosin patch. Any clues to get more info and solve the problem?