We don't throttle individual sites on the CGI server, and we would advise of any bandwidth problems before we did anything.
I don't have any obvious suggestions for this, if one site is running and the other isn't. Have you checked your scripts are functioning normally?
I'm not to investigate if I can, but this woulden't immediately seem like a server problem.
www.domainname is available now, although you need to point your whole domain name to the CGI server then, which does mean that you can't use your normal non CGI webspace with the domain. To do this set up the following records via the portal:
www CNAME cgi.plus.net.
Then raise a ticket on your account asking us to set your domains www service to "Semi Automatic". This should take around 2 hours to take affect.