Has this been reported by anyone else? This is the first time I have heard of an issue with 990fx and Win8.1. Since 8.1 has been available since late last year I find it very hard to believe that AMD does not have support for it. They work closely with Microsoft on all major releases. If they have not released a new driver specifically for 8.1 there is no need for it. It sounds more like a bios issue to me. Or your board battery is crap. lol This just doesn't seem right to me.
AMD Chipset Drivers Latest chipset drivers.
ASUS Windows 8.1 - Ready Motherboard : Get you Onboard Asus claims they have done all the testing for it to be fully 8.1 supported. Are you on the latest bios?
If you type in 990fx and Windows 8 or 8.1 it is amazing how many times you will see that board listed but no others. Just that board.
I want to ask all of the overclockers I know if they have ever came across this issue and did they find a fix.