Well, based on that comprehensive description of the problem and your setup, I can confidently confirm that it could any one of a million things.
As a first step to getting the problem proven and some element of diagnosis, head over the Thinkbroadband.com and setup an account, and then create a Broadband Quality Monitor. Let that run for 24 hours, and post a link to the chart here. That'll show us whether there's a problem with your connection, and we can proceed from there.
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***** If you think my answer has helped - please provide me with a Kudos rating and mark as Helpful Answer!! I do not work for Virgin Media - all opinions expressed are of my own and all answers are provided from my own and past experiences. Office 365, Dynamics CRM and Cloud Computing Jedi