Inconsistent performance

To whoever can assist,

I am working with a decent sized database on an extremely powerful machine.
The specs follow:

OS: RedHat Linux 9.0
PG Version 7.3
Memory 1 gig
CPU Quad Processor - Unsure of exact CPUs
Hard Drive 80 gigs
Database Size 2 gigs

As you can see the server is built for overkill.

The problem that I see is as follows.

I do a rather simple query: select count (*) from large-table where column
= some value;

About 80% of the time, the response time is sub-second. However, at 10% of
the time, the response time is 5 - 10 seconds.

This is nothing readily apparent at the system level that comes close to
explaining the performance hits. CPU and memory usage (as measured by top)
appear to be fine.

Although there are certain tuning issues within the database itself, no
documentation I have seen seems to indicate that tuning issues would lead
to such inconsistent response time.

Any ideas?

Regards,

Joseph