I have a site dedicated to Rey Mysterio.

The Site URL is http://reymysterio.us

But when I type http://www.reymysterio.us, It sometimes give me page not found. Why is this error when I add that extra www. Is there a solution to this probelm

Do you have a "www" handle registered in the DNS of your domains ?
Or a catch all ?

If not, it is that the domain name without machines name (the www) is defined, but www.reymysterio.us is not, thus the request to fail.

The simplest to do is associate a cname record (a shortcut) from www to your main adresse.

I did not understand your solution dude. Can you explain me more elaborately how to go about doing it. Also I noticed that sometimes it works and soemtimes it does not

Well, there is not much to do.

A url is composed of a subdomain (www), a domain (reymysterio) and a tld (us)

In the DNS of your registrar, the domain name and the tld are set, but the subdomain www may not be.
So, if it's not, you should login on your registrar account and add one.

Now, if it works sometimes and sometimes don't, it could be a problem with the server hosting your site that there is a problem.

Each request to a domain don't trigger a query on your regitsrar DNS server.
In fact, you use your isp dns resolver, when you ask a page, if he has alerady tho domain in his cache, it won't go further.
If not, it will ask the servers recusrsively until they find the ip address of the subdomain.domain.tld you are asking for.

So, a problem can occur in the resolver of your isp, in the dns server of your registrar, on the server hosting your files, and anywhere in the middle.

Now, when I check your domain whois record, I see that the dns server are set to

Name Server:                                 NS3.NPSERVE.COM
Name Server:                                 NS4.NPSERVE.COM
Created by Registrar:                        DIRECT INFORMATION PVT LTD DBA PUBLICDOMAINREGISTRY.COM
Last Updated by Registrar:                   DIRECT INFORMATION PVT LTD DBA PUBLICDOMAINREGISTRY.COM

For me, from Switzerland, there are around 20 servers between me and them, but they respond ok.

But I het a timeout when I'm asking the ip address of your www.reymysterio.us server:

~$>host www.reymysterio.us
;; connection timed out; no servers could be reached

This is for reymysterio.us without www in front:

~ $>host reymysterio.us
reymysterio.us has address 198.145.40.51
Host reymysterio.us not found: 2(SERVFAIL)
Host reymysterio.us not found: 2(SERVFAIL)

What seems strange to me here, is those 2 "not found" lines.
Looks like there is a round robbin between multiples server set up, but that some servers don't answer.

I'm not a absolute expert on DNS (just handling mine, to be flexible and to learn how), so I can't say.
But you should ask the firm who is handling your dns what happens.
There seems to be a problem on their side, in my opinion.

The short answer...

It is a hosting issue

I accept ,

i also experienced the same problem for my domain http://karthikeyan.net.in

But not now