So there are only the 2 computers on the network right?
You did not get any kind of new router or anything did you? I know one of the routers I have had some kind of uni cast option that seemed to broadcast to all pc's allot and would wake up a pc.
For the hell of it, have you tried to make it sleep with a dummy network, Just a switch and a manual IP(this way the network card is still running). This would ensure the network card is not just waking up with no activity.
WOL can be a pain at the best of times.
I did a quick search for you and saw this, may be worth a shot. At least as long as it does not prevent the wake you need.http://jack.ukleja.com/fix-for-windows-7-random-wake-from-sleep-problem/