Well, as a Network Admin, I know a good bit about TCP. I can tell you that, if I take my laptop to work, plug it into our 20MB Fiber Internet connection, and leave the 300KB/s limit set in JDownloader, the SAME THING happens. I can log into my Cisco firewall with it's ASDM manager software and bring up a line graph of the internet connection and see that I'm not even using more than about 2MB of the 20MB connection (That's all computers on the network combined).

But, if I then push the limits in JDownloader up to say, 2,500KB's and restart it, then the bully download will run up to 300-400KB/s, and not cause the other host to lose bandwidth. And, I see the bandwidth on the firewall go up, sometimes to 5, 6 or even 7 MB.

With them running just fine like that, I can bring the bandwidth limit in JDownloader back down to 300KB/s and the problem starts again.

So, the problem is NOT an ISP, or TCP problem. It's how JDownloader allocates the bandwidth you allow it.