Can't tell from your description of the problem, but you might want to check to see if your NIC is set to "Allow this device to wake the computer." I created a new image for our Optiplex 755's and I forgot to uncheck this box. As a result no matter how much I fiddled with the Power Management GPO, they wouldn't stay asleep until I cleared that checkbox on each machine.
After going to an MS Technet seminar on workstation deployment and evaluating WAIK, WDS, etc. from Microsoft, I just ended up sticking with updating our Ghost licenses. Ghost just seemed so much easier and less involved than the Microsoft solutions.