Slow Downloading Speeds?

I have a 512 kbps connection. When i check my downloading speed on speedtest.net, it's approx 70 kBps. But while downloading files I sometimes don't even get a speed of 20 kBps and sometimes it's normal. Please provide me a solution for this as i'm not getting what I'm paying for. And please don't explain me the differences between kbps & kBps. I'm fed up of it. Also don't tell me the gpedit.msc trick. Please answer quickly.!!