"with www" or "without www", that is the question!

i have created a new virtual server to my domain by v.m.
at first days (2-3) it only works with www. and donot work without it after these days every thing changes and it works without www and do not work with www. this is my zone file : can any one help me?

$ttl 38400
@ IN SOA new.ezzi.net. root.new.ezzi.net. (
1195663948
10800
3600
604800
38400 )
@ IN NS new.ezzi.net.
mydomain.com. IN A 73.21.250.50
www.mydomain.com. IN A 73.21.250.50
ftp.mydomain.com. IN A 73.21.250.50
m.mydomain.com. IN A 73.21.250.50
mail.mydomain.com. IN A 73.21.250.50
mydomain.com. IN MX 5 mail.mydomain.com.
ns1.mydomain.com. IN A 73.21.250.51
ns2.mydomain.com. IN A 73.21.250.52

my apache configs:

<VirtualHost 73.21.250.50:80>
SuexecUserGroup "#502" "#503"
ServerName mydomain.com
ServerAlias www.mydomain.com
DocumentRoot /home/mydomain/public_html
ErrorLog /home/mydomain/logs/error_log
CustomLog /home/mydomain/logs/access_log combined
ScriptAlias /cgi-bin/ /home/mydomain/cgi-bin/
DirectoryIndex index.html index.htm index.php index.php4 index.php5
<Directory /home/mydomain/public_html>
Options Indexes IncludesNOEXEC FollowSymLinks
allow from all
AllowOverride All
</Directory>
<Directory /home/mydomain/cgi-bin>
allow from all
</Directory>
</VirtualHost>

now its not working with www but dig result from local server is:

[root@new ~]# dig mydomain.com

; <<>> DiG 9.3.1 <<>> mydomain.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 38921
;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0

;; QUESTION SECTION:
;mydomain.com. IN A

;; Query time: 3 msec
;; SERVER: 72.9.108.146#53(72.9.108.146)
;; WHEN: Thu Nov 22 00:18:15 2007
;; MSG SIZE rcvd: 32

[root@new ~]# dig www.mydomain.com

; <<>> DiG 9.3.1 <<>> www.mydomain.com
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 47816
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 1, ADDITIONAL: 0

;; QUESTION SECTION:
;www.mydomain.com. IN A

;; ANSWER SECTION:
www.mydomain.com. 38400 IN A 73.21.250.50

;; AUTHORITY SECTION:
mydomain.com. 38400 IN NS new.ezzi.net.

;; Query time: 9 msec
;; SERVER: 72.9.108.146#53(72.9.108.146)
;; WHEN: Thu Nov 22 00:17:42 2007
;; MSG SIZE rcvd: 78