I too sometimes have noticed that o2 have sluggish DNS servers. How this manifests is that browsing might feel sluggish when you browse a site, but once it's going on a download or similar it flies along.
I'd agree with the others, connect a cat-5 cable between your computer and the router & disable your wifi. The o2 router should have shipped with a yellow cat5 cable about a metre in length.
Also, try and collect the connection statistics - robertos' website will guide you through that if you should require.
Was on Xilo C&W LLU on MRCEN, Now Be* on MRARD.