slow dns query

Hi, look the first result of this command:

root@naveserver [/usr/local/assp]# dig cakemusic.com | grep "Query time:"; dig cakephp.org | grep "Query time:" ; dig caraudiopoint.com | grep "Query time:" ; dig caroil.com | grep "Query time:" ; dig terra.com.br | grep "Query time:" ; dig uol.com.br | grep "Query time:"
;; Query time: 125 msec
;; Query time: 164 msec
;; Query time: 112 msec
;; Query time: 149 msec
;; Query time: 154 msec
;; Query time: 143 msec

At the second time, 5 seconds after the first:

;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec

but after 2 minutes, this is the result:

;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 0 msec
;; Query time: 49 msec
;; Query time: 143 msec

The 2 last queries, by the hight time, I can supose it wasn't cached more.

How can I do to optimize the cache?

my resolv.conf:

domain mydomain
search myhostname
nameserver myeth1

thanks,

Andre.