Large table performance

Hello -

I have a fairly large table (3 million records), and am fetching 10,000 non-contigous records doing a simple select on an indexed column ie

select grades from large_table where teacher_id = X

This is a test database, so the number of records is always 10,000 and i have 300 different teacher ids.

The problem is, sometimes fetching un-cached records takes 0.5 secs and sometimes (more often) is takes more like 10.0 seconds

(fetching the same records for a given teacher_id a second time takes about 0.25 secs)

Has anyone seen similar behavior or know what the solution might be?

any help much appreciated,
Mark

ps. My shared_buffers is set at 5000 (kernal max), and work_mem=8192