Q dns won't work after some time

[Jason]

Hi,

My DNS will be working fine if I retart my Linux box, however, after some
time period, the DNS seems dead, I don't know why. Then if I do nslookup
from the outside dns server, it can't find my A record.  My linux box is
RedHat 7.3.
Here is my zone setting:

$TTL 3600
@ IN SOA DNS.XXX.com.  Jason.XXX.com. (
   2003060403 ; serial
   3600 ; refresh
   3600 ; retry
   3600 ; expire
   3600 ; ttl
   )

@ IN NS DNS.XXX.com.

@ IN MX 5 DNS
XXX.com. IN MX 1 DNS

DNS IN A 9.9.9.9
localhost IN A 127.0.0.1
XXX.com. IN A 9.9.9.9

www IN CNAME DNS.XXX.com.

Any help will be appreciated.

Jason