In agreement with Ian Kemp, I would rather suspect driers and configuration. When you change slot, that usually causes windows to decide it is another device.
So the usual of removing completely all graphic drivers and then installing again would perhaps fix the issue. If it was me I'd first try to remove the card from device manager
then restart and let windows install it again. Then do the drivers reinstall.
Also install all latest drivers from motherboard vendor.
Check your bios settings, maybe disable integrated graphics if possible. If you don't know about BIOS, maybe resetting to defaults will make sense.
I doubt a hardware failure of motherboard might be it. At least far less likely. PCIe lanes are negotiated between device and motherboard so a hardware issue will more likely result in being unable to see the device at all. But who knows.
2Have you referred to the manual to verify your using a PCIex16 slot? – Ramhound – 2020-01-29T20:39:34.660
I know it's the right slot because it used to work perfectly well in that slot. It has only recently started having problems. – Cutter – 2020-01-29T20:41:29.550
2Primary slot / secondary slot is pretty vague. Are you referring to the one closest to the CPU when you say Primary Slot? Have you checked the BIOS settings to be sure you're in x16 mode? – Natsu Kage – 2020-01-29T20:46:29.720
Yeah the one closest to the CPU. It's definitely the slot that it SHOULD be in. But recently I've been having the problems described in the question with that slot, meaning I've had to move to one further away. – Cutter – 2020-01-29T20:48:57.170
Just googled for an image of that motherboard, have you tried using the on-board HDMI port? – SpacePhoenix – 2020-01-30T10:23:10.037
If problems started recently, then I would suspect either hardware damage or software issues - perhaps a static discharge or similar damaged one or more PCIe lanes on either the GPU or the CPU, causing issues with the PCIe link; or maybe a bad driver version is causing trouble. – alex.forencich – 2020-01-30T23:52:40.733
@alex.forencich. Thanks for this. I've got another GPU coming next week, it will be interesting to see if that fixes the problem. The fact that I also now seem to be getting much milder audio issues with the non-GPU audio output suggests that it might be the mobo. – Cutter – 2020-01-30T23:58:03.730
@Cutter, What CPU and version of Windows are you using? If you could tell us your BIOS version too that would be helpful. This info may clear up any confusion about whether those IRQ assignments have anything to do with this or not. – Romen – 2020-01-31T00:12:24.757
i5 4460, Windows 10, BIOS Version/Date - American Megatrends Inc. 0902, 08/05/2014. Also added to specs in question. – Cutter – 2020-01-31T00:29:58.487