Web forwarding problem

[#1]

Hi,
I currently have 2 domains registered with 123 Reg that use web forwarding to point to the Plusnet servers. One site is on the CGI platform, the other on the regular platform.
http://www.simari.co.uk
http://www.simonleetranter.co.uk
I can access both websites with no trouble here in the UK, but I have some friends around the world who often have difficulty accessing them. The sites simply refuse to load up and they get a time out error. It's always the case that both sites are inaccessible at the same time.  Checking my webstats, I notice that both sites have a large number of logged 404 Errors.
If I provide affected users with the web addresses that point directly to the Plusnet servers, bypassing the web forwarding, they can access the sites with no trouble.
I thought it must be a problem with 123Reg, but they assure me nothing is wrong and they have no errors on their servers. So, I now think it must be a problem with Plusnet. Does anyone have any ideas? Any advice would be greatly appreciated.
Cheers!

[#2]

I've tried to access the website from home and using a proxy (based in USA) both gave me the same websites. so I couldn't replicate this from a proxy base in America.
Both domains are registered with 123-Reg but each domain has different Name Servers?
Domain: www.simonleetranter.co.uk
  Registrar:
        Webfusion Ltd t/a 123-Reg.co.uk [Tag = 123-REG]
        URL: http://www.123-reg.co.uk
    Relevant dates:
        Registered on: 16-Jul-2002
        Renewal date:  16-Jul-2010
        Last updated:  01-Jul-2008
    Registration status:
        Registered until renewal date.
    Name servers:
        ns.hosteurope.com
        ns2.hosteurope.com
Name Server Info:
IP address: 212.67.202.2
Host name: ns.hosteurope.com Registrar: TUCOWS INC.
canonical name: ns.hosteurope.com
212.67.202.2 is from United Kingdom(UK) in region Western Europe

Domain: www.simari.co.uk
    Registrar:
        Webfusion Ltd t/a 123-Reg.co.uk [Tag = 123-REG]
        URL: http://www.123-reg.co.uk
    Relevant dates:
        Registered on: 07-Oct-2008
        Renewal date:  07-Oct-2010
        Last updated:  28-Oct-2009
    Registration status:
        Registered until renewal date.
    Name servers:
        dns1.freehostia.com
        dns2.freehostia.com
Name Server Info
IP address: 66.40.52.47
Host name: dns1.freehostia.com  Registrar: ENOM, INC.
canonical name: dns1.freehostia.com
66.40.52.47 is from United States(US) in region North America
I'm not sure if you are hosting your domains with different provider (not 123-Reg) or do 123-Reg Outsources other companies for hosting or web forwarding?

[#3]

Thanks for looking into it for me Rami.
Earlier this week, both sites were inaccessible in parts of the US, France, Dominican Republic, and Italy for a whole day. So, I went ahead and moved the simari.co.uk site to a different server (Freehostia), so that's why the nameserver is now different. The problems I was having with that site seem to have gone now.
The other site is still hosted on Plusnet and is still using web forwarding from 123 Reg, but if the problems keep occurring, I may have to move that one too. 123 Reg tell me everything is fine their end, and I never get problems accessing the site from the UK, so I'm stumped as to why the site is sometimes inaccessible in some parts of the world. Is it possible that your servers block access from certain countries during busy periods?

[#4]

Hi,
You've mentioned that the  www.simari.co.uk is working fine now after changing the name services, while the other domain is still with ns.hosteurope.com and ns2.hosteurope.com
These 2 servers are the authoritative servers for your domain. so Plusnet DNS servers have nothing to do with this process. I'm not aware of any blocks to our webservers, especially that you are able to access the website from anywhere using www.username.plus.com
the best way to determine this is to do httptrace to your domain from US or another country where your domain is not accessible.

[#5]

Thanks Rami. I'll give your suggestions a try.  It does seem that 123 Reg are at fault here, even though they're telling me nothing is wrong.