How to find the culprit in server load spikes?

Hi everyone,
     I host a Postgresql server on Ubuntu 12.04 and I am facing server
load spikes (if I run top, it goes up to 25-30 on a 4-core system)...
In some cases, I have to restart potgresql service because users call us
complaining of the slowness, but in some cases I can leave things on
their way and I see that after a bunch of minutes (about 5-10) the
situations drops to the normality (0.50-2 load).
The problem is, as in the most cases, the I/O, but I need a small hand
to know some methods or tools that can help me to investigate who or
what is causing me these spikes.

Any help would be appreciated.
Best regards,
Moreno.