Okay, finally got the new identical RAM kit delivered, and had time to run some tests, I think the conclusion is indeed a faulty RAM kit:
First I ran a positive control test where I reset the BIOS settings to Auto for all items I had changed in a previous post, and re-installed the faulty RAM in the A2-B2 slots
--> Crashes as frequent as before, as expected
Then I placed the faulty RAM in the A1-B1 to see if it could be a faulty slot (mobo) issue instead
--> Crashes as frequent as before, consistent with the faulty RAM theory, but does not rule out the faulty mobo
Third step was to remove the faulty RAM and install the new RAM kit in slots A1-B1
--> Not a single crash in a total of 10 hours of gaming, where I used to have crashes every few minutes. Still consistent with the faulty RAM theory, but does not rule out the faulty mobo
Last step was to move the new RAM to the A2-B2 slots to test them:
--> Not a single crash in over 18 hours of gaming/intensive use.
So in conclusion I think it is safe to assume the mobo is fine and the old RAM kit is faulty, so I'll send it out for replacement.
Here are my lessons learned on this one:
- Even though everyone hopes for a quick fix, ALWAYS perform test steps one by one (e.g. remove ONE supposedly faulty component and test right away). If you change multiple things and the issue is resolved, you won't know for sure what actually solved the issue.
- MemTest results are definitely not 100% accurate unless you leave it to run long enough. I think it's pretty safe to say that running it for less than 10 passes is worthless.
Note that even with enough successful passes (20+ have been advised) it does not replace real-life situations a.k.a. whatever you usually do on your computer that makes it crash.
- If the component you're suspecting is plugged in the motherboard, don't rule out the possibility that the motherboard itself is faulty, ideally have a negative control test with another similar component that is known to work fine
- RAM issues are a pain to diagnose as you can easily have false negatives as I have had. Also there's nothing you can do with your computer without RAM
I'll mark this one as resolved, hopefully it can help other people.
Anyway, thanks a ton for your super fast follow up and very knowledgeable answers.
TL;DR: The RAM was borked in the end, tadaaa