I'd assume I'd have to manually set the timing of both to match the slower of the two?

Although honestly, if I have 16GB working without issue, do I really even need an extra 8GB? Having 24GB...

No good. Despite passing the test, it'll POST but then automatically restart. It works fine with just the new memory in.

Would this be because of the timing differences? 9-9-9-24 and 11-11-11-28

Popped in the new sticks that came in today in slots 2 and 4, along with the sticks I left in there from last night in slots 1 and 3, and it POSTed on the first attempt. Memtest is running now.
...

I'm kind of at my wit's end right now. :(

I had this issue when I first built my PC a few years back but ultimately had it "resolved" until last night

The motherboard is very, very weird about...