Well, I have been having pc issues aswell, big time lag and I just couldn't understand why, so I cleaned it and defragged it, did system restore, cleaned it, ccleaner, avg scan it, man nothing, i close alot more tasks nothing, i tired changing to higher settings and less themes being used still laggy, I restart my pc again for the 1millionth time and well, I think it has something to do with avg as it's running fine and when I close it it happens, not the fact this has anything to do with your problem but atleast you no your not the only one, well wait, now mines fixed hahaha lol, joking man, but this is pissing me off also about your problem
EDIT: Why not try just unhooking your router and try with a direct connection.