DNS not resolving

Hello,

I have a setup in which access is give to 3 customer and dns is listed as below.

Domain name servers: 10.20.x.x, 10.213.x.x,193.128.x.x

DNS search list: a.com, abc.com, zx.local

All customers are using access zones to access Isilon

ghost# isi zone zones list

Name   Path

------------

System /ifs

zeeco  /ifs

chgka  /ifs

bbdc3  /ifs

------------

Issue is AD goes offline on some nodes sometimes and if we do nslookup and traceroute dns name is not resolved.

Workaround

if we move ip address of that customer to first and then do isi auth refresh AD comes online and issue is resolved and after some days AD goes offline for other customer.

How can we get a permanent fix for the issue ?

moving ip address is not a permanent solution.

moreover we do not see any specific error in logs.

I need some input to resolve the issue.

Thanks

Chughh