why the dns is so slow?

wsm

I add :
nameserver 202.96.209.5
nameserver 202.96.209.133
to my resolv.conf. But these days I find the browser always takes 10-30 seconds to "looking up XXXX.com". Once it get the address it became fast. And in terminal the nslookup gives address very very slow.
Is there any one knows why? Or I should change the dns server to some other one?