Performance issues with postgresql-8.4.0: Query gets stuck sometimes

Hi,

We are using postgresql-8.4.0 on 64-bit Linux machine (open-SUSE 11.x).
It's a master/slave deployment & slony-2.0.4.rc2 is used for DB
replication (from master to slave).

At times we have observed that postgres stops responding for several
minutes, even couldn't fetch the number of entries in a particular
table. One such instance happens when we execute the following steps:

- Add few lakh entries (~20) to table X on the master DB.

- After addition, slony starts replication on the slave DB. It
takes several minutes (~25 mins) for replication to finish.

- During this time (while replication is in progress), sometimes
postgres stops responding, i.e. we couldn't even fetch the number of
entries in any table (X, Y, etc).

Can you please let us know what could the reason for such a behavior and
how it can be fixed/improved.

Please let us know if any information is required wrt hardware
details/configurations etc.

Regards,

Sachin