Recently happened with me: all my downloads (I tried Chrome and Mozilla) terminate after a while because of unknown network error. Pretty irritating, isn't it? After several hours I found the problem which was my anti-virus. I use ESET Smart Security and it does port checking by default. Disabling this option (Advanced Setup -> Web and Email -> Web access and antiphishing protection -> HTTP, HTTPS -> Uncheck Enable HTTP checking) permanently solved my problem with downloads.
I doubt that it is a good idea to disable it permanently. So, I turn it off every time I need to download something larger than 10 MB.
