Argh. I have the same problem, and have had for some time now: check out my graph. These are over several weeks, you can clearly see a divide. I'm not sure. Also, these are only in the time since Blueyonder became Virgin Media. Before that I never had a single problem. Now, I'm getting not only these incredibly slow speeds frequently, but I'm also getting lagged out of, and disconnected from, online games which is far more frustrating.
my internet is meant to be 4Mb.