I don't no how to give you the information you need to diagnose this problem but ill give you as much as i no.
Im running a 2 pc lan off a Belkin 4 port Modum/Router.
Adsl2+ and had a techncian come look at the line a cple months ago becaz a little noise.
He said the connection at my house was crossed or someshit didnt really pay much attention and he fixed it.
Before my problem was the internet would drop out constiantly like every 5-10 minutes it would drop oout.
now it's i get huge lag spikes every 3-4 minutes.
I'm playing Counter-Strike Source and bam like 700 Ping, for a steady 30 seconds then it drops down and same thing in another 5 minutes time.