100x slowdown for nearly identical tables

I have two tables that are nearly identical, yet the same query runs 100x
slower on the newer one. The two tables have the same number of rows (+/-
about 1%), and are roughly the same size:

db=> SELECT relname AS table_name,
db-> pg_size_pretty(pg_relation_size(oid)) AS table_size,
db-> pg_size_pretty(pg_total_relation_size(oid)) AS total_size
db-> FROM pg_class
db-> WHERE relkind in ('r','i')
db-> ORDER BY pg_relation_size(oid) DESC;
table_name | table_size | total_size
----------------------------------------+------------+------------
old_str_conntab | 26 GB | 27 GB
str_conntab | 20 GB | 20 GB

Both tables have a single index, the primary key. The new table has
several more columns, but they're mostly empty (note that the new table is
SMALLER, yet it is 100x slower).

I've already tried "reindex table ..." and "analyze table". No difference.

This is running on PG 8.4.17 and Ubuntu 10.04. Data is in a RAID10 (8
disks), and WAL is on a RAID1, both controlled by an LSI 3WARE 9650SE-12ML
with BBU.

If I re-run the same query, both the old and new tables drop to about 35
msec. But the question is, why is the initial query so fast on the old
table, and so slow on the new table? I have three other servers with
similar or identical hardware/software, and this happens on all of them,
including on a 9.1.2 version of Postgres.

Thanks in advance...
Craig

db=> explain analyze select id, 1 from str_conntab
where (id >= 12009977 and id <= 12509976) order by id;

Index Scan using new_str_conntab_pkey_3217 on str_conntab
(cost=0.00..230431.33 rows=87827 width=4)
(actual time=65.771..51341.899 rows=48613 loops=1)
Index Cond: ((id >= 12009977) AND (id <= 12509976))
Total runtime: 51350.556 ms

db=> explain analyze select id, 1 from old_str_conntab
where (id >= 12009977 and id <= 12509976) order by id;

Index Scan using str_conntab_pkey on old_str_conntab
(cost=0.00..82262.56 rows=78505 width=4)
(actual time=38.327..581.235 rows=48725 loops=1)
Index Cond: ((id >= 12009977) AND (id <= 12509976))
Total runtime: 586.071 ms

db=> \d str_conntab
Table "registry.str_conntab"
Column | Type | Modifiers
------------------+---------+-----------
id | integer | not null
contab_len | integer |
contab_data | text |
orig_contab_len | integer |
orig_contab_data | text |
normalized | text |
Indexes:
"new_str_conntab_pkey_3217" PRIMARY KEY, btree (id)
Referenced by:
TABLE "parent" CONSTRAINT "fk_parent_str_conntab_parent_id_3217"
FOREIGN KEY (parent_id) REFERENCES str_conntab(id)
TABLE "version" CONSTRAINT "fk_version_str_conntab_version_id_3217"
FOREIGN KEY (version_id) REFERENCES str_conntab(id)

db=> \d old_str_conntab
Table "registry.old_str_conntab"
Column | Type | Modifiers
-------------+---------+-----------
id | integer | not null
contab_len | integer |
contab_data | text |
Indexes:
"str_conntab_pkey" PRIMARY KEY, btree (id)