I, as many others seem to, have had similar issues. I'm not very tech minded, so my "fixes" were to re-install windows 8.1 completely. It worked, for a week at best... However my internet issues aren't just related to browsers.
I've downloaded Titanfall at least 3 times now... 50gb game is a pain to have to re-download constantly... When my internet decides to play up, I won't even be able to log into Origin, let alone download anything. Yet, after reinstalling windows it would usually start running at 3 mbps... Of course, when these issues are occurring, my browser seems incredibly slow, and usually can't load any pages at all, not even google. At the present, it seems only certain pages are slow to download, and Origin etc won't work. After reading everyone's comments, I'm going to try unchecking IPv6, and hope to god it works. But with the frustrations I've had so far, I'm not giving it high hopes.
Also, has anyone else been having issues turning their pc off when experiencing these issues? When mine starts to play up, occasionally I'll "Shut down" my computer, everything will close and all communications are cut, but the pc keeps running, the lights and fans are constantly running. I've only had these issues since upgrading to 8.1 (Computer is pretty darn good, and only 5 months old)