In our forums a lot of Virgin ADSL users appear to be experiencing problems with slow or intermittent downloads. For at least some people it appears that tweaking the TCP/IP settings and mainly the Maximum Transmission Unit (MTU) will improve things.
We must stress that you do these changes at your own risk. We welcome feedback from users of the Virgin service in their dedicated forum section, so that others can see if the tweaks do work. Also the forums are a good place to ask questions if you are unsure what to do.