Why do you say you need to wait an hour?
I can't speak to what is common practice in pcbuilds but if time is a factor I expect the issue to be thermal, especially if it is working and then stops. Check everything that has to do with power and use any available method to check for hot components. Any experienced electronics hobbyist will usually just do touch tests on chips, electrolytic capacitors, and inductors if you have decent access to board level components. Most GPUs have all the stuff covered in nonsense cosmetics. You just need to touch something grounded so that you don't have a static charge potential before touching board stuff.
I'm not saying this is your issue or anything, but it is common for through hole components that experience a lot of heat cycles eventually developing solder joint issues especially if the original manually soldered joint was bad. I would look for any power wire connection issue first.
The first rule of troubleshooting is "thou shalt check circuit ground." If there is some kind of real delay, I would cause the issue, and then immediately probe between the card and motherboard ground to check that they are still connected. Then I would put a 10A rated multimeter in series with ground and check the current to the card before and during the fault. Don't go down this path until people experienced with more pcbuilds chime in, this is just general electronics 101 type info.