Optimizer too eager to choose full table scans

Here's the same query with slightly different paramers. The optimizer chooses
to use a full table scan for the one set of parameters. Even though it's only
10% more records the query takes 4 times as long to execute, presumably
because of the full table scan. (Yes, I ran these several times to reduce disk
caching effects.)

Is there a parameter to adjust to tilt the scales somewhat back in balance
here? It seems to be overeager to use full table scans.

slo=> explain analyze select count(*) from ad_dept where dept_id between 730 and 738;
QUERY PLAN
----------------------------------------------------------------------------------------------------------------
Aggregate (cost=5417.18..5417.18 rows=1 width=0) (actual time=875.74..875.74 rows=1 loops=1)
-> Seq Scan on ad_dept (cost=0.00..5409.53 rows=3059 width=0) (actual time=0.18..869.45 rows=2767 loops=1)
Filter: ((dept_id >= 730) AND (dept_id <= 738))
Total runtime: 877.81 msec
(4 rows)

Time: 879.80 ms
slo=> explain analyze select count(*) from ad_dept where dept_id between 731 and 738;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------------------------------
Aggregate (cost=5236.00..5236.00 rows=1 width=0) (actual time=232.25..232.25 rows=1 loops=1)
-> Index Scan using ad_dept_dept on ad_dept (cost=0.00..5229.06 rows=2778 width=0) (actual time=0.40..214.76 rows=2477 loops=1)
Index Cond: ((dept_id >= 731) AND (dept_id <= 738))
Total runtime: 232.42 msec
(4 rows)

Time: 237.53 ms

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greg