Postgres locking up?

I'm experiencing a problem with our postgres database.  Queries that
normally take seconds suddenly start taking hours, if they complete at
all.

This isn't a vacuuming or analyzing problem- I've been on this list long
enough that they were my first response, and besides it doesn't happen
with just a single query.    Turning off autovaccum (and switching to a
weekend vaccuum) seems to have reduced the frequency of the problem, but
did not eliminate it.  Besides, I've seen this problem with copy
statements, which shouldn't be that susceptable to problems with these.

Nor is it a problem with normal database locking- when it happens, I've
been poking around in pg_locks, and nothing seems wrong (all locks have
been granted, no obvious deadlocks).

Restarting the database seems to help occassionally, but not always.

This is happening both in production, where the database is held on an
iscsi partition on an EMC, and in development, where the database is
held on a single 7200 RPM SATA drive.  Both are Opteron-based HP 145
servers running Centos (aka Redhat) in 64-bit mode.

What I'm looking for is pointers as to what to do next- what can I do to
track the problem down.  Any help would be appreciated.  Thank you.

The output of pg_config:
-bash-3.00$ /usr/local/pgsql/bin/pg_config
BINDIR = /usr/local/pgsql/bin
DOCDIR = /usr/local/pgsql/doc
INCLUDEDIR = /usr/local/pgsql/include
PKGINCLUDEDIR = /usr/local/pgsql/include
INCLUDEDIR-SERVER = /usr/local/pgsql/include/server
LIBDIR = /usr/local/pgsql/lib
PKGLIBDIR = /usr/local/pgsql/lib
LOCALEDIR =
MANDIR = /usr/local/pgsql/man
SHAREDIR = /usr/local/pgsql/share
SYSCONFDIR = /usr/local/pgsql/etc
PGXS = /usr/local/pgsql/lib/pgxs/src/makefiles/pgxs.mk
CONFIGURE = '--with-perl' '--with-python' '--with-openssl' '--with-pam'
'--enable-thread-safety'
CC = gcc
CPPFLAGS = -D_GNU_SOURCE
CFLAGS = -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Winline
-Wdeclaration-after-statement -Wendif-labels -fno-strict-aliasing
CFLAGS_SL = -fpic
LDFLAGS = -Wl,-rpath,/usr/local/pgsql/lib
LDFLAGS_SL =
LIBS = -lpgport -lpam -lssl -lcrypto -lz -lreadline -ltermcap -lcrypt
-lresolv -lnsl -ldl -lm -lbsd
VERSION = PostgreSQL 8.1.4
-bash-3.00$