Windows 10 is quirky sometimes. I had a heck of time getting my son's Z97 based computer to be stable. All sorts of crashes, sudden reboots, etc. Luckily, over the next several months Microsoft released updates that fixed the issue. However, I spent a good month trying to figure out the issues. You can try setting it to a different sleep in your BIOS, and maybe that could be the issue?
As far as the 8 year old PSU, that came with a 5 year warranty, and there are two trains of thought here about putting a older working PSUs in a new build.
One side says it's perfectly fine, and as long as it's a quality PSU, when it finally goes, the safety mechanisms in the PSU will prevent it from damaging your hardware. They will say "That's why you paid a pretty penny for a nice unit in the first place".
The other side will say that is most likely will happen when it dies, but there's always a small chance it could damage something, so why risk it?
I'm kind of in the second group, but I see group one's points. You definitely got your return on the investment (8 years of use on a 5 year unit), so why chance it?
There's no right and wrong here, it simply cones down to what you are comfortable doing. But in the case of your components, since it isn't a $500 CPU, or a $300 motherboard, I'd check the voltages it was putting out, and if they looked good, I'd probably continue using your PSU. As PSU age (and from heat), they begin losing some of their output capacity.