I have a theory as far fetched as it may seem.
I have recently had constant access to virgins 60mb cable broadband and im both impressed and unimpressed.
Firstly, when its fast, its very fast indeed and its great.
However there's a good percentage of the time that its quite slow and has quite a lazy feel. Now i've looked around the street and there's not that many folks seemingly tied into the green box at the end of the road and so it can't be an issue of over crowding.
The network is massively capable and i've always been a fan of it.
The only way us folks can test our bandwidth is to use the likes of speedtest.net etc. However I have done tests when the Internets been a bit lazy and its still give me a 50mb+ speed and it just doesn't tally up with the experience online.
I have a theory where the virgin servers have filters and know that a user is accessing speedtest.net and are 'dumping connections' and transferring bandwidth to the user at that time, so that it appears to speedtest that you are actually running at full speed.
Dan