So, I've had this issue (they are actually 2 issues) since 2/27/2011. After months and months of testing and money spent on a new power supply and a UPS neither of which fixed the issue, I'm here to report back on my findings. I have logged my testing and have quite a bit of data by now.
There's no formal fix for this issue but there's a workaround and I came to a few conclusions:
1. Wake from sleep becomes unreliable once the PC is asleep for more than 2 hours.
2. The longer the computer sleeps, the more likely it is to have problems waking up.
3. FG is the least unstable BIOS and the only bios produced that fixes the (second issue) USB3 issue mentioned here:
http://forum.giga-byte.co.uk/index.php/topic,3866.msg63791.html#msg637914. The FH bios re-introduces the aforementioned issue (USB3 hybrid sleep resume after powerloss fails)
My workaround:Setup a scheduled task that wakes the computer every 2 hours. I set it to e-mail me, so I know it's happening. It then goes back to sleep 5 minutes later.
It sucks that Gigabyte never really fixed this but at least my computer has not failed to wake up this way since March 12 2012.
Good luck!