Problem with Download speed in my Dell XPS L521X Laptop

Hello, I have Recently Purchased a Dell XPS L521X laptop from Ebay. It was fine up to few days ago, then all of a sudden the download speed changed strangely it starts normal from about 500kbps then drops down to 0kbs and then again rises and drops down to 0kbps and then goes up again. I tried to search some option but it's no use. 
I did the ping test and showed the Request timed out for many times. 
Did somebody encountered the same problem? I am running on Windows 7 Home Premium. Also I installed Malwarebytes and removed some malwares but still the same issue appears. Please help ASAP.