I pay for a 2mb unlimited line, with EFHBroadband and a speedcom router.

Just recently I've been getting problems with my broadband. It happens when I download large amounts.....It typically tends to be 4gb files split into managable chunks from usenet. I start off with a 180kps d/l speed, which slowly reduces itself down to little more than a 56k modem. This also affect http sources, but not ftp, port 21 is fine.

I've submitted the fault to my isp, but as yet they can't find anything wrong

It stinks of port throttling to me........How can I proceed with this?