Wake on LAN - light sleeper!

Wonder if anyone can give me any guidance on how to solve an annoying but non-critical problem.

I've a network at home with a desktop running Linux (kubuntu Gutsy), two laptops running XP and a networked printer. (All connected via ethernet). I use a router to connect to the internet. They all have a fixed IP address on the LAN.

I've got both laptops set to wake-on-lan, and have ticked the option for "Only a management machine can switch on this computer" which I believe stops any old network traffic from causing a wake-on-lan. I can wake any of the laptops by sending a magic packet - this all works ok.

The problem is, one of the laptops keeps waking up randomly. I think it must be something to do with the network card. If I run an 'nmap' command from the linux machine that will wake up this laptop (but not the other). If I disconnect the ethernet cable it doesn't happen, so I know it must be something to do with network traffic. Sometimes it just wakes up for no reason - I'm not sending any network data at all.

The only port I have open on my router is 22, which allows me to access an ssh server on the linux machine.

Can anyone give me some pointers on what sort of things to look at to try to sort this out?

Thanks