Firstly, let me say that I have no personal experience in this area, so my suggestions are "second hand" based on conversations with those working in this field, and a lot of guesswork.

Also, they are based on ADSL, rather than cable broadband, but the principles might be the same?

I would suggest that you look at resources usage on the server. Not the number of customers on at any one time, but the bandwidth that they are using.

It strikes me as interesting that the problems are during office/working hours, rather than in the evening, when students are playing games and downloading stuff

I know that my ADSL provider works on a contention ratio of 50:1. That is the resources are potentially shared between 50 users. Now, my contract says that I may connect one computer (at any one time) to the service.

If I set up a network and and attached to the service via a router, I do not think that my ISP would know? all they would see would be a lot of traffic/bandwidth usage coming from the router?

I wonder if there are a number of unscrupulous business users who have registered for the service as home customers, but are actually running networks through it? My friends who work in this area, have certainly encountered this.

I cannot quite remember the daily bandwidth allocation that I am allowed, but I do know that if there were 50 users (contention ratio is 50:1, remember) and they all used their allocation in the same time window (business hours or evening) it would bring the service to its knees.

Now, each user would not exceed their allocation, individually; so probably would not show up in the logs, but the combined peak usage would cause a problem.

I know that these services are set up to satisfy "average" demands, so creating an unusual peak could cause these problems.

Just a thought