Internet Speed incremeant

Hello,
I'm using the bsnl broadband connection with a speed of 256 kbps but its not downloading at the particular speed. But in some websites like centalmusiq.com i downloaded a particular file at a speed of 1.5 mbps. I was amazed but after some time i downloaded the same type of file and i noticed the speed it showed 7.4 - 9.4 kbps. Why this occurs ? Then how can i increase the speed of download. Which increases and decreases the speed of download either the software like DAP or the websites or the time of download or the browser. Please give me a solution. I'm in Chennai (Taml Nadu).