CSM has nothing to do with the problem.
Hidden device means no longer detected, but windows has drivers ready if it shows up again.
Any Asus board I have owned in the bios detects the graphics card regardless of the make. (example: gigabyte 2080ti is detected in my current m/b)
It is possible that power supply cables to the motherboard (above the CPU, to the left) or directly to the GPU are causing issue (have spares / modular psu could swap them out / make sure connected in the saddles firmly). But it does sound like evga 980 may have an issue. If you could try the graphics card in another system might confirm it's not the problem or the source of the issue.
Hope you find this of help.
Rule out Windows 10: (I doubt it is the problem)
run as admin [powershell] (type into search bar, when you see it pop up, right click on it and run as admin)
wait till complete
now run (still in powershell)
Dism /Online /Cleanup-Image /RestoreHealth
wait till complete
if either found and fix integrity errors or corruptions, reboot and then repeat both steps again.
if you get stuck in *loop (*greater than twice) of either of them them needing to repair/fix, this indicates the OS is unstable and unable to be properly repaired, and it's would be time to bite the bullet and consider *reset of the OS (*which may also fail) or a fresh install.
Check you are running the latest motherboard bios as this can stop windows updates
you can run [CMD] then [ver] to check but this is current version Microsoft Windows [Version 10.0.18363.535] 64 bit
Ok, did both. Got no integrity errors or corruptions for either.
Ok, so today I booted up the PC. Went into my HDD version of Windows. Suddenly...there's the card in Device Manager. I haven't touched the PC since last night. Simply switched it on this morning. Will be interesting to see if it's in the SSD version of Windows. And I'm not holding out that on re-boot or shutdown I'll have the same problems again, but wtf! Was there a Windows update or something?
Booted into SSD drive version of Windows and no card. The SSD version of Windows has been updated with .NET Framework version 1909 (KB4533002) and (KB4530684). The version on HDD drive hasn't been updated with these. I'm wondering if these updates might be the issue.
Odd behaviour, let's try changing out the CMOS battery with a new one and see if still persists. Sometimes they can cause very odd behaviours as they die.
You can stop worry about windows 10 causing this, it's most likely a hardware issue you have.