Site not reachable by www.xxxx.com, only by xxxx.com - random

[alteredsanity]

My site is reachable only by xxxx.com and not its www.xxxx.com. This happened yesterday for about 12 hours and has started today again.
No changes have been made in the dns zone on the server or at the registrar.
I try to restart bind and apache and no changes.
Whats happening?
Help

[alteredsanity]

Heres some DNSstuff:
Searching for alteredsanity.com ALL record at d.root-servers.net [128.8.10.90]: Got referral to C.GTLD-SERVERS.NET. [took 6 ms]
Searching for alteredsanity.com ALL record at C.GTLD-SERVERS.NET. [192.26.92.30]: Reports ns1.alteredsites.com. [took 7 ms]

Domain Type Class TTL Answer
alteredsanity.com. NS IN 172800 ns1.alteredsites.com.
alteredsanity.com. NS IN 172800 ns2.alteredsites.com.
alteredsanity.com. NS IN 172800 ns1.alteredsites.com.
alteredsanity.com. NS IN 172800 ns2.alteredsites.com.
ns1.alteredsites.com. A IN 172800 66.36.242.226
ns2.alteredsites.com. A IN 172800 66.36.245.92

I've made no changes to the dns what so ever within the last 6 mos, and can't figure it out.

[IberHosting]

Hello,

you need to create a CNAME or A entry for your domain, www.domain.com pointed to domain.com

Like: www.domain.com CNAME domain.com
or
www.domain.com A xx.xx.xx.xx with your IP.