I'll probably wait until F12 official for now. I tried raising PM_* voltages but no change, WHEA error will still come after some undetermined amount of time. Strangely I did notice that power plan seems to impact this... if I use a more aggressive plan like 1usmus, then it seems much less likely to happen.I'd give a try to F12a; I used F11 for quite a lot and at least for me it's more reliable.
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Best way to test stability is gaming with some medium to high workload; something too heavy will not trigger that easy instability.
My suspicion is that there is some power saving going on when system is idle that triggers WHEA error... the one thing I haven't tried is the option in the CBS -> XFR menu that disables the SOC from doing any power saving... I may try that later this weekend.
Otherwise RAM tests all pass, I can run any type of benchmark/games for 10+ hours and never see an issue... as long as I don't let system sit in Windows idle for extended amount of time. Ex. yesterday I played Doom Eternal for like 5-6 hours... exit game, go to other room and come back and WHEA error has triggered while sitting at Desktop.
None of this occurs if I just lower FCLK to 1867.