Don't know if this might be of use to someone.
After many months of the annoying sleep and immediate re-wake problem, I've found a simple solution...
With the BIOS set to APCI Suspend Mode as 'Suspend 3 only' (don't know if that is actually essential), change the USB Wake Up jumper USBPW9-14 (the one by the 3 USB connectors) to 2 and 3 (centre and right) instead of the default 1 and 2.
I actually have all 3 of those USB connectors un-used (using only the backplate ones) so can't say if having them empty is relevant.
So for me, with that now solved (faultless sleep, either manual or timed, and instant wake at any time with keyboard), everything is working perfectly.
Hope this might be of help. (Maybe even for other similar boards)?