It's a strange phenomena that, I had a session where if the phone rang the adsl line reset. turned out to be a corroded connection up a pole, took ages for the eng to find it.
Oddly today I did a large download from radipshare (8 100MB files) the first started to come down @ 98KBs instead of my normal 700+ so I just carried on adding to the downloads until I had all 8 coming down together, but they were all coming down @about 98 giving me my normal download speed of 7 - 800. So obviously it wasn't my line speed that was the problem.