Disk latency goes up during certaing pediods

Hi list,

I am running Postgres 9.1 on Ubuntu 12.04. I have a dedicated disk for
pg_xlog, using ext4 filesystem with journaling in writeback mode
During high load times, the disk usage is arround 40%. The IO write time is
constant at about 3ms. On certain occasions roughly once in 15 days, the
IO write time goes up to about 10ms. This makes the disk usage go up to
almost 100%, probably saturation, and the INSERTS DELETES UPDATES run
considerable slower than normal.This lasts for about 2 hours and then the
latency goes back to 3ms and everything is normal again.
Has anyone seen this behavior? What could be causing the increase in
latency?

Thanks!

Germán Becker