Your problem may have been fixed with overclock. It may have not been getting enough power to begin with and therefore flickering black. It could have also been the fans with the temperature but that is less likely.
My recommendation is too raise the power limit to as far as it can go, this won’t effect every day power use or the overclock, it just ensures that it won’t happen again.
With the bad pcie slot, that may just have been a bad cable which can cause problems usually further down the line of constant use or right after adjustment. This all happened to me awhile ago and it was due to me not plugging one of the power connectors into my GPU fully hence the similar experience in general.
Best of luck and I hope it stays fixed.