Thanks for your quick reply!

I am not sure about my OC, due to i just flash the bios, with the bios of somebody else! and the rig works for more than 7 months

I don't think that temperature is the issue, due to, the rig if working now with 2 GPU at the same temperature around 74c.

there is any way to test the power supply?, it means, it look ok, but it look that when the rig demand load, the crash occurs

thanks
                                                                                    
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76C seems a little warm for start up. What OC do you have these set to? How about undervolting?

Turn the rig off, leave it off for 2 minutes. Turn it on. See if that helps. If it doesn't set the cards in question (that are not working) back to default speeds and see if that works, then OC slowly from there.
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