Incompatibility is not something that could be there from the first moment.
You say you've had your system running for months, in those months various of changes have occured.
- Windows updating with the monthly patches, sometimes earlier due to a critical problem,
- Programs updating,
- Settings changing,
- New programs installing, new drivers present on the system,
- New devices, new drivers,
This is a small list of examples for changes that could be there for someone in months.
Each change makes a system run differently because different code needs to be executed when a change is being made.
It is just a matter of time before enough changes are made to have an unstable system.
Besides, Windows 8, 8.1 & 10 are Operating Systems that use the BIOS more than the previous ones. Because your motherboard drivers & BIOS aren't updated anymore for newer OS, it takes a single change to get the system unstable. We are a few months further since the thread was created, many changes have been applied. Some may or may not have made things worse.
You could try to update to Windows 1607, the newest version of Windows 10 that has been released 6 days ago, to see if stability gets improved. Though I would recommend to replace some hardware, upgrade your motherboard at least where you possibly also need a new CPU due to compatibility.
You could also try to install Windows 7 or 8.1 on your system, a trial, and see if any problem occurs in there (best is on a Windows without any 3rd party software installed). Simply create a new partition from the free space of either of your partitions, temporary remove the other drives, install Windows on this partition and see how it goes.
This may not be without any risk, recommendation to backup important data to an external drive before doing this. It sometimes causes problems