Wake on LAN issue

Have a interesting problem which I cannot find a solution.

Have one computer on my network that refuses to stay asleep will wake as so as it does. Have checked all the normal problem areas USB, Timers etc everything seems to be working, the other 10 PC's on the network all basically configured the same work fine.

For one last attempt I isolated the problem PC from the network only having the ADSL router connected to it still the same problem reconnected to the network bingo sleep, yeah fixed it. Bugger found I had no internet forgot to power on the router, Mmmm no sleep instant wake. Fine problem found bad router changed to new one bugger me problem still occurring.

What and why could make a router keep a computer awake, while all others stay asleep