Slow SELECT on small table

Hi,

I am using Postgresql: 9.01, PostGIS 1.5 on FreeBSD 7.0. I have at
least one table on which SELECT's turn terribly slow from time to time.
This happened at least three times, also on version 8.4.

The table has only ~1400 rows. A count(*) takes more than 70 seconds.
Other tables are fast as usual.

When this happens I can also see my system's disks are suffering.
'systat -vm' shows 100% disk load at ~4MB/sec data rates.

A simple VACUUM does *not* fix it, a VACUUM FULL however does. See the
textfile attached.

My postgresql.conf is untouched as per distribution.

Can someone hint me how I can troubleshoot this problem?

Thanks!

Martin