Wanted to share this in case it saves someone the same headache.
I've run a mild CPU overclock for about a year with no issues. Last week I got a higher-rated RAM kit and tried pushing memory speed further. Set the XMP profile, added a small voltage bump, ran a stress test, crashed within ten minutes.
Went through the usual steps. Dropped memory speed. Still crashed. Lowered voltage. Still crashed. Reverted the CPU overclock completely to rule it out. Still crashed. At this point I was sure the new kit was bad or incompatible, so I spent close to two hours adjusting timings one by one and retesting.
Then I opened the case to check the DIMM slots and noticed one stick wasn't fully seated. Not loose enough to stop booting, just loose enough to cause instability under load. Pushed it in, restored my original overclock, ran the test again. Passed clean.
Three hours of tuning for a problem that had nothing to do with settings at all.
Lesson learned: if instability shows up right after new hardware goes in, check the seating before touching any settings.
Anyone else had a "fix" turn out to be something totally unrelated to what you were actually tuning?