Sudden peaks in load and disk block wait
Solution 1:
The problems were related to a incompability issue between Ubuntu 8.04 LTS (Hardy) and the Dell PERC 6/i RAID controller, as reported in this bug: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/607167 Upgrading to Ubuntu 10.04 LTS Lucid (kernel 2.6.32) resolves the issue.
In case anyone else runs into the same issues.
Solution 2:
May be you are running some heavy query which is doing a full table scan. Have you checked your slow query log.
If thats case just add proper indexes.
PS: Sorry If you have done this already.