I know you said at the top the power supply checks out, but do you mean it's of sufficient size or that you've constantly monitored the levels it's actually outputting at the point of the crash?

Reason I say that it is because I had the same problem myself and that I started to get random restarts/lockups which got worse over time and it turned out to be the PSU even though it "looked" fine.

Even though it was of sufficient capacity (1200W known brand and I rarely actually used over 400W) it was 6 years old and it seems it just went bad over time (apparently not that uncommon) so when the graphics card/processor had a drastic jump in power requirements, the PSU couldn't always keep up with that sudden change. It started off at occasional reboots, but got worse over time, until the point 6 months later that it was even crashing as it tried to boot Windows. Thankfully as soon as I put a new PSU in it fixed the problem and I haven't had a single reboot since!

If you have another PSU (yeh I know, who has a spare PSU?) I think it'd be worth your time to connect it up to see if it fixes the problem. Sorry if this isn't any help though, but just wanted to mention it because I had this myself I assumed that because the PSU seemed to be working/was big enough it must be okay and I couldn't have been more wrong!