On Windows 10 64- bit, R5 1600, Crosshair VI Hero (non-wifi), R9 290, evga G2 750w, G.Skill 3200MHz RGB Tridents.

Over the past few days I have switched from a 120mm CM AIO to a full loop (360mm rad, D5 pump), mostly Barrow/Bykski parts. No leaks, pumps work, fans work, everything works from the outside.

Since the CM AIO has the exact same mounting holes as the R9 290, I have bolted them together using Nylon M3 nuts and bolts, and put heatsinks, thermalpaste, with direct airflow (3 bottom intake fans on the in-win 303) over them, just like how Gamers Nexus do their ghetto hybrid mods. Didn't want to get a ~$125 GPU block since I was planning to upgrade the GPU soon, and it will be added into the full loop then.

The crashes happen like this: All monitors go black, lose video signal, half a minute later the board turns to Q Code 8 with orange LEDs, but half the time it just stays on code 24 and green lights, which should be normal operation. No amount of auto-restart or using the RESET button will save it now, the Q code will either be 8, 0d, or 24. The system has to be fully powered off, then started up again, where it will do its usual 3 'fake boot' cycles before actually booting.

Old (tested stable) OC settings were P-stated to around 3.8 Ghz and 102.2 Mhz bclk, RAM 32x. Voltages were near stock, I stopped at those OCs because any further would require much higher voltages that I weren't even comfortable with testing. Ran those OCs for about 4 months without a problem.

I have tried this so far:
1. Updated Bios (from 1501 to 3502)
2. Reset ALL OC and other bios settings to defaults
3. Reinstall GPU drivers with DDU and 18.2.1
4. Scanned for viruses
5. Attempting to sfc /scannow but it's giving me problems, I'm following a user's solution to DL Windows 10 thru Media Creation Tool to fix DISM, but the system usually crashes before a complete DL and it can't be resumed.

The crashes happen randomly, I do monitor the temps and they've pretty much never exceeded 45C. I've also been touching the VRM/mosfet heatsinks after each crash to check if those overheated, and they have always been only slightly warm to the touch, if at all, so definitely not in the ~100C range.
However, playing games or streaming videos trigger this crash much more consistently. Watching downloaded videos on SMPlayer never crashes the system. Overwatch, for example, always crashes during hero select. Streamed videos will crash the computer in a few seconds, even if I just load the tab, pause the video, and let it buffer. The fact that it's in the background buffering is enough to crash the computer.

Anyone have any suggestions/ideas on what might be happening? Is it an unfortunately-timed Windows update screwing me?