I had a similar problem with my Netgear router.
Only solution for me was to replace it with a model that had better throughput.
So I bought a FVS338 and sold my FVS318 on ebay.
My connection is capped at 10Mbps but now I get the full 10Mbps http://www.newegg.com/Product/Product.aspx?Item=N82E16833122007&Tpk=fvs318http://www.newegg.com/Product/Product.aspx?Item=N82E16833122148&Tpk=fvs338
You won't need one of those. I needed one because of VPN connections.
Google the problem and see if there is a source of information about your router's throughput.