Throttling?

I know this sounds like throttling but I'm not sure.

Here's what happens.

I start downloading some TV show from Itunes. The first 2 or 3 shows download fine. Then when it gets to about the 3rd or 4th download it starts to slow down and then eventually just stop. The problem is it then freezes my entire internet access. So it's not really just iTunes at that point, it's all my network devices getting their internet connection frozen. I then re-set my modem and then it's all good again.

If I then start the process again (the downloads) the internet on all devices will eventually freeze and hang up after about the 3rd or 4th download.

What is this?