head scratcher or am I missing something obvious

[cd34]

It still sounds like your hosting company is writing a firewall rule to drop his traffic when it exceeds some connection limit. When the rule is triggered there are several different things they can do. Redirect to another site saying bandwidth exceeded, refuse connections, or, drop the traffic altogether for a period of time.

Some of the firewall rules we use (not for web), look at the number of connections within a minute. If that is exceeded, traffic from that IP is dropped until 60 seconds of inactivity from that IP. If they continue hitting every 30 seconds, it refreshes that timer. That sounds pretty similar to what you're describing.