Downlaods (and heavy traffic apps such as youtube) have the same problem on bothg computers. I called the Xtra Help Desk and they found nothing wrong, even told me I had a fast connection. I have a hard wired ADSL splitter in the house (as I have a monitored burglar alarm and Sky).
The Xtra Tech suggested there was a problem on the actual computer. I have since done an OS reload (clean build) on one of the computers and while it is running quicker now (there was a lot of rubbish on it) the download problem still occurs. Both computers running Windows XP Home SP3.
I have also swapped out my ADSL Router/Modem from the 6-year old PureTek to the Thomson Model that Telecom are issuing, no difference, although I had been emaning to do this for a while as this old one needed constant rebooting.