reset bios Motherboard removing battery.
No luck, unfortunately.
How does the system go when logged into safe mode? does the monitor stay up there
How long can the monitor stay up when your int he bios? this could point to a S problem.
Are there any system logs under event viewer, look for critical or error around the time of the monitor going black,
maybe try Plug in the 750ti with the rx480, once the screen goes black plug the monitor into 750 and look at the event viewer, nope the motherboard only has only one PCIe, ok maybe the black screen is the inlet HD taking over the OS.
latest BIOS update, old but still strange,
- Improved AMD Graphic card compatibility.
- Improved memory compatibility.
- Support PSC B-die DDR3 Memory
- Fixed Realtek lan still can work after disable "Onboard LAN Controller" in BIOS.
- Updated VBIOS and GOP Driver.
- Hide item "IGD Multi-Monitor" when "Initiate Graphic Adapter" set to IGD.