The GPU is good, but you're getting errors on all the GCWs and some of the SGSs. You've already done the obvious troubleshooting steps.
Clearly there's *something* wrong with the hardware. If this is a new computer, and you bought the computer already assembled, I'd think about returning it. If you built the computer, it's time to start reseating components. It's possible there's a bad connection or a dirty contact somewhere. When you have the parts out, look carefully at the connectors. There may be a bent pin in there somewhere.
If reseating components doesn't work, it's time to start swapping components if you're comfortable with that. While you might not have a spare CPU and motherboard sitting in the parts bin, it's possible that the power supply is bad. Swap in another power supply if you have one that's suitable. Even though there's no indication it's at fault, swap out the GPU for another. If possible, remove memory sticks one by one and see if the problem goes away with a specific memory stick.
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