Gradual increase in CPU utilization by postmaster

We are running Postgres 7.0.3 on a Linux machine with a 2.4.0 kernel.

We have noticed that as we run our application which does
a lot of updates that CPU utilization (by postmaster) gradually
increases as time passes.

We have been able to reproduce it with the following
shell script which we use to feed a continuous stream
of updates to the DB:

-----------------
#!/bin/sh

while ( true ) ; do

echo "update ruser set cname='xyz' where userid='abcdef'; "

done
-----------------

We connect it to psql to send the updates to the DB as follows:

./psql_perf2.sh | psql -U AAA DBNAME -q

When we start the script, the "top" command shows about 75%
CPU utilization but this gradually increases to 97%
in a few minutes.

Could anyone suggest a reason for this behavior?

Thanks
R. Anand