I read https://www.cnet.com/forums/discussions/maximus-viii-freezing/ and came away you need to make it simpler. Mixing GPUs and older GPU as well is known for such hangups, EVEN when you think the GPU is not in use.
You're living with a locker so it's time to try simpler. Pop out the old 570 and keep testing.
I recently reassembled my PC.
I put RAM from single channel to dual channel and reset BIOS to default. Note: The dual channel wasn't working before. Now the PC is freezing after about 3 hours of gameplay. It was mining cryptocurrency for about 2 days in row without any issue. Before the reassembly, everything was working fine. I have encountered 2 types of freezing. One with code AO, which cuts out sound and my backlit keyboard goes black, screen is frozen. The PC fans are still spinning. The second with code 04 is with loud buzzing sound, screen also frozen. I found out that I can temporary fix it by reseating CPU cooler for a day or two. I also found out that 3/4 of cooler standoffs in my PC don't have the black plastic ring under them (I don't know if it's a problem). I ran OCCT Power supply stress test without issues and also the memtest.
It shouldn't be a GPU issue because it was running fine before the reassembly and is running fine for cryptocurrency mining which is like stress test for GPU. My temps are CPU - 50°C underload
GPU 75°C underload
CPU: i5 6600k, no OC
GPUs: rx 570, doing nothing while gaming, GTX 1070, no OC while gaming
RAM: Corsair Vengeance LPX 3000MHz
MoBo: Maximus VIII hero
PSU: RM 650x