Cable Forum - Connection speed drops and picks up.

Hello!
I originally had a 2 meg connection, which was upgraded for free to 10 meg a few years ago. I didn't have any problems with it, until I started downloading large files (like games from Steam).
The download speed starts off fine, doing around 1.1mbps. After about 40 minutes, the download speed drops to a solid 250kbps and does not go up again.
Interesting thing is, I've checked my internet speed using speedcheck.net.
First test, the day after my download slowed, these were the results:
Download: 9mbps. Upload: 0.98mbps.
Same day, when downloading at 1.1mbps for about an hour, speed dropped to 250kbps. Speed test results:
Download: 2.5mbps. Upload: 0.99mbps.
Download speed picks up again after about 5 hours or so (not sure about the exact time). Same thing happens if I try to download files from any other website.

It seems that my internet connection 'reverts' to the original speed that I was getting before the free upgrade after downloading for a bit. What is the cause of this?