There are some things that are very hard to test for. For example, failing VRMs or capacitors (MB, video card, PSU, etc.). You've also mentioned that the Seasonic you are using is heavily used, so it's possible that while it's still generally providing clean power that it is occasionally allowing either insufficient voltage or "dirty" power through.
If you haven't done so yet (I can't remember) you can try setting everything to stock (so, no overclocks even "factory" overclocks). There's always a potentially for a slightly less than stable driver or TSR (old terminology) but something that is resident in memory that's fucking things up after a certain amount of time or after it's been doing stuff for a certain amount of time.
Try to keep an eye on whether it does it after doing a certain set of actions in Windows. For example, a program or series of programs or a certain time after a program has been run, etc. Intermittent locks after a long period of time (days/weeks) can be extremely difficult to track down.
Regards,
SB