Traffic management on fibre?

[#1]

I'm having issues with newsgroup downloads.  My connection's pretty stable at 37 mb/s but at seemingly random intervals newsgroup downloads won't reach any more than 240 kb/s.
I've tried with two different news providers and get the same result.
Is this a traffic management thing or something different?

[#2]

At certain times it is limited to that on Value Fibre
http://usergroup.plus.net/prodcomp1.php

[#3]

That's a really helpful link, thanks 
Think I'd better start downloading overnight instead!