New computer possibly causeing internet to drop out

It seems to be more than coincidence that when I connect my new custom built computer to the internet router via LAN cable, that the internet starts slowing down and dropping out for my computer and all other computers connected. Downloading seems to be faster, but websites load a lot slower and from time to time I have to reset the BT Voyager 2100 internet router. There were never any problems until I plugged the new computer in and I have set the MTU to 1500 and used a tweak program to try and improve things as well. The same thing happened when I used a wireless adapter. To begin with it claimed that connection to the router was good but that there was limited or no internet connectiveity and all the correct lights are light on the router. I have tried with different internet browsers also. The motherboard I'm using is the BFG Nforce 680i as it is that that everything is connected too.