I have a very similar, if not the same problem... I'm actually trying to build an XP/MCE NAS/server based on an Intel DG965WH motherboard with the Intel onboard Gigabit ethernet controller. I'm not interested in using magic packet WOL, but I am interested in the system waking from S3 when an attempt is made to access one of its shares - so that's the Directed Packet. The system is set to go to standby after 5 minutes of inactivity...
Although I can get the system to wake quite happily from S3 on accessing one of its shares, at the most I can only seem to manage about 30 seconds of sleep, usually much less, before it pops awake again. If I disconnect the network cable it sleeps fine, reconnect it and it wakes up (regardless of whether there are any other systems active on the system with a share open). I'm in a constant wake/sleep/wake/sleep cycle, with sleep being anything between 1 and 30 seconds.
I guess either something is trying to access the PC legitimately based on its IP/MAC address whilst its asleep and the issue needs to be stemmed there (I can assume only my router, as it happens with nothing else on the LAN?), its waking on just "general" network activity or for some other reason.
Not even sure how to look into this... I've tried with a packet sniffer attached to a different PC on the network, but I'm not really sure what I'm doing here; it doesn't throw anything up with either the problematic machine's IP address (which is fixed) or MAC address at the point it sleeps/wakes, but I'm not convinced I'm seeing all the network activity...
I've two other threads open
here and
here, but as yet without reply...