Postmaster only takes 4-5% CPU

Hi all:

When I run a (big) query from pgsql, postmaster used to take almost 100%
of CPU. Now, with aparent no reason, Postmaster only takes 4-5% CPU and
the same query takes really longer to finish.

I have tried renicing -20 postmaster but it still only takes a small
percentage of CPU, while 90% is idle.

This is a "top" screen while running the query:

2:27pm up 5 min, 2 users, load average: 0,80, 0,26, 0,08
74 processes: 72 sleeping, 2 running, 0 zombie, 0 stopped
CPU states: 3,1% user, 2,1% system, 0,0% nice, 94,6% idle
Mem: 1031012K av, 246040K used, 784972K free, 0K shrd, 11496K
buff
Swap: 2040244K av, 0K used, 2040244K free 196916K
cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
1165 postgres 0 -20 3476 3476 2912 D < 4,5 0,3 0:02 postmaster
268 root 15 0 0 0 0 SW 0,1 0,0 0:00 kjournald

Any idea of how can I get postmaster to eat as much CPU as possible?

Thanks a lot! Ruben.