System hangs when Insert/Update

Hello,
I am using PostgreSQL 7.0.2 on RH 6.2

The DB works smoothly in the past 8 months but until recently, the
system would hang when doing insert or update statements.

When I use
ps aux
I found a lot of processes like
postgres localhost xxx xxx INSERT
postgres localhost xxx xxx SELECT
postgres localhost xxx xxx idle
are running

When I killed one process using
kill -TERM pid
all those processes gone and the DB returns to normal. But after some
insert/update, the problem exists again...

I tried to use strace on the process and found that they seems waiting

for something since the following statements are execution repeatedly
select (0, NULL, NULL, .....
Maybe they had been fall in some infinite loop..

Could you give me some help ?

Best regards,
--
Lee Kwok Shing (***)
http://www.geocities.com/lkslee.geo