Might be your ISP (Internet Service provider) having checked on your monthly bandwidth use and decided to shape (throttel) the amount you are able to use at any specific time or in general over a period of a month.
I assume you've tried the things suggested in this and other threads regarding slow download speed (i.e. deleting the LW preference folder, updating Java with the newest JRE offline installer 1.5.0_07, trying to download a magnet link, etc.)
If not, you should try these things first before calling your ISP to ask whether they indeed shape your bandwidth (don't be surprised if they deny it; would be bad for puplicity)
