Periodic bottle-neck in high load app

Hi,

we're running a high load applicatoin (Disks are hard working). While the
load is rather constant in it's short term behaviour, we are experiencing a
bottle-neck in regular intervals.

In such case all connections get used up and pgadmin's
"server-status"-Window is totally filled with statements, while single
statements take 3-10 sec (normal: <0.2s). But then there're long periods of
constant average load wich postgres is rather playing.

SO: Is there some periodic scheduled activity, postgres does lets say every
5 min ? (that can pond up many many statements)
What else could do that? Currently all periodic activities are paused.

Tanks in advance

PS. new harddisks are coming