No rows returned when rows are there!

I have been using Postgres for several years now and am running quiet a
large system, with a sustained number of users around 50, in constant
use. I am using 7.1.3 on a Linux 2.2 Kernel. I have had no real problems
until now. Recently, at arbitary times in the day, "all" queries return
no rows. I do not get any error messages on the server or on the the
clients, they execute the query just fine but no rows are retuned. This
happens on serveral unconnected tables, and seems to be system wide.
This lasts for a fews seconds (10-60) then starts working again.

This is very worrying as no errors seem to be generated, and it put the
integrety of the whole database in jeopardy.

Does anyone know what's happening or how I can get some idea of what is
actually happening?