Bob -
This isn't about bandwidth at all. It's about the number of connections to your site at any given time. No site can have unlimited connections or it would have the ability to overrun the server and shut down other sites.
Each account has a limited number of connections per second. Isolating your site (which was done) helps protect you from other sites so that you are not sharing resources with other customers. If you have scripts that fail to close or if you are having many hits per second to your site that are maintaining a connection to the site indefinitely, then that can affect the performance.
You also share the resources between 2 websites. You are not limited in your bandwidth to the sites, but the sites do share the connections that are allowed. I have increased the connections as much as I can.
We usually encourage customers to make sure that scripting sessions close properly. In all reality - your site would probably run faster/more efficiently on our Linux cloud servers, but we don't offer unlimited bandwidth there. Those packages are newer and handle the connection requests better.
Your site is visited by many bots and spiders and those could be tripping you up.
Bandwidth is not the problem at all for your site - it is the sheer number of hits and connections to the site that you are getting. That is what is causing the errors that you see. Your site is out of resources as far as the amount of visitors it can accomodate per second.