I had the exact same problem show up on my sheeva this past weekend. Loadavg shooting up to 6-7 and pdflush pegging the CPU. I had made no changes in the software or config so it was most likely a hardware issue. I was still running off the default stock Ubuntu image from the NAND flash that came with the sheeva. When i checked 'dmesg' i saw four messages complaining about 'bad ersaseblock' on the NAND flash and the bad eraseblock addresses reported were in the root filesystem (/dev/mtdblock1) partition. Suspecting that was the problem i copied over the root filesystem image to a 2 GB SDcard and changed bootparams so that root is mounted from the SDcard. This link was immensely helpful. http://plugcomputer.org/plugwiki/index.php/SD_Card_As_Root_File_System
. That solved it! I am no longer using internal NAND flash for my root filesystem and loadavg is back to < 1 with the sheeva responding normally while running all my applications (asterisk, openvpn etc). This link is also very helpful if you want to boot off USB or SDcard (i.e. have your kernel read from those devices) and have your root filesystem as well on USB/sdcard http://computingplugs.com/index.php/Booting_entirely_off_an_external_USB_device
. I guess i should go all the way and also boot off the sdcard instead of just having root on it but for now its working great and the original problem is gone.