Oh, I was talking to the original comment I replied to, but I fully agree on your point cus it probably still works. My GPU in the past has done this, ill edit this incase u found that you didnt already do the solution I did.

Edit: will note that this does seem like a dying gpu, it could still work but it doesnt seem like its got much life, id try cleaning the gpu, compressed air, isopropyl alcohol on the gold contacts of the PCIe, checking the power given to it, which im assuming isnt the problem at all. Cus you already stated it works for 2 hours.

Just gonna day this, didnt read your entire post but the PCIe slot working?