Hi, This has always been a major problem, about 90% of the time when im downloading, my download speed will rocket down from 3 mbps to like 10 kb/ps And then I can't even download the file, I Do have a router connected but I use a modem, I am on virgin media and im using google chrome.
Depending on what else is going on with your network, that could affect your speeds. If other computers are connected, if you are browsing the internet, playing a game, etc. This can all affect the speeds.
Also, depending on what you are downloading from it could be an issue. Downloading torrents will vary. Downloading from a dedicated server should provide the best results.
You can use toast.net and use the ISP speed test to see what your throughput is to compare to other ISPs as well.