strange performance anomalies

Hi,

We recently put up a new 7.2.1 installation on Solaris 8 that serves a
24x7 e-commerce site. The system seems to run pretty well most of the
time but we see a consistent form of performance anomaly.

Watching pg_stat_activity the system spends most of it's time running
smoothly with queries clearing through sub-second. We have a production
job we run which immediately sent the site into a tailspin though.
Starting that job caused hundreds of select statements to queue up in
the pg_stat_activity view. This seems odd since MVCC would lead us to
believe that shouldn't happen. Readers shouldn't block wholesale like
that unless we're using DDL on the table or doing a vacuum per the
online docs at
http://www.ca.postgresql.org/users-lounge/docs/7.2/postgres/locking-tables.html

Nevertheless, turning off foreign key constraint checking via:

update "pg_class" set "reltriggers" = 0 where "relname" = tablename

cleared up the problem, load fell off to normal in seconds. So how is it
that fk contraints apparently cause what look like table-level locks?
Or, at the very least, cause a heck of a lot of select statements to go
into a holding pattern for some reason?

(On a side note, it seems the current locking behavior might also
violate the I in ACID by causing an independent delete transaction to
actually "see" or be affected by the content of the transaction using FK
constraint locking before it's clear that the transaction will commit).

At any rate, being somewhat new to tuning at this load level for PG I'm
not sure if I'm supposed to be tinkering with max_lock_per_transaction
here. Could this be evidence of a lock starvation issue or something?
Guessing here and any input would be appreciated. Thanks in advance!

ss