Disk latency goes up during certaing pediods

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