First of all, I am sure you are aware of the fact that the DX50SO board is long (long!) past its end-of-life date and long out of interactive support. Secondly, because Intel also exited from the Desktop Boards business, I am sure you are aware of the fact that there have been no resources available to look into compatibility issues for quite a few years. Finally, I am sure that you are also aware that the 1st Generation Core and Xeon processors and the 5 Series chipsets are not supported for Windows 10 (in fact, neither are the 2nd generation Core processors or the 6 Series chipsets). The last version of Windows supported on this board was Windows 7.
Bottom line, there could be - and likely are - Windows 10 compatibility issues in the DX58SO BIOS and they certainly could play a role in the sleep state not working properly. The only thing I can suggest (well, other than returning to Windows 7) is that you ensure that you are running on the latest available BIOS release. As for why this problem only recently appeared, it is likely because Microsoft has been re-writing code right and left and creating these compatibility issues in the process (and worse, when any are uncovered, they just shrug their shoulders and drop support for the environment(s) in which they occur).
I too have a DX58SO system that is still alive and running well under Windows 7. With updated SATA controllers and an SSD, it still has plenty of oomf (though it may be a bit of a power hog doing so). I use it as a backup development machine. I will retire it sometime in the near future. Thoughts on a good home for it?
Hope this helps,