Query time

Hi,

I think im specting problems with a 7.4.8 postgres database.

Sometimes some big query takes between 5 to 15 seconds. It happens
sometimes all the day it does not depend if database is busy.

I have measured that sentence in 15 - 70 ms in normal circunstances.

Why sometimes its takes too much time?
How can I fix it?
Is a postgres version problem, database problem or query problem?

Any ideas will be apreciatted.

Ruben Rubio