It still could be the fans, if they are providing wrong rpm information the card could end up in some weird state.
I’d still remove the fans and the plastic shroud from the card and connect some other fans (e.g. your noctua case fans) to it to see if the card lets them spin constantly. You can use cable ties to fix at least two of such fans to the cooler.
All at your own risk, but I try some troubleshooting anyway.
Are you sure that it wasn’t 475? 