What about peripherals? Mouse, Keyboards, etc, have you tried disconnecting them?
If they are causing this, you can turn off the waking of the computer in the USB setting of Windows.
What FastKeys does is just tell Windows to sleep the screen, and this seems to be working just fine. If it is waking again after some seconds goes beyond the scope of FastKeys, in my opinion, and should be treated as a problem with other variables, like devices, windows settings, etc.