Why does my internet speed keep dropping?

[Annoyingcloth675]

okay so I'm on Virgin Media and I'm meant to be getting 30Mbps download speed but lately my internet has been dropping very low, sometimes down to 10Mbps
http://s7.postimg.org/wnsi5azjf/lollo.png
I understand that 20Mbps is still very good, but in the small green graph at the bottom it's dropping, instead of a smooth, flat speed at 30. I can't even play a full game on Xbox live, and its very laggy in the party chat. even going onto a webpage takes a way lot longer to load up on all devices. I have tried to restart my Super Hub a million times now but still nothing as changed, it still drops every few seconds.

Any feedback appreciated