making sure the cooler is sitting flat & making good contact with the GPU die is crucial, also the entire GPU die needs to be covered with paste
if you replaced thermal pads on the card - they need to be the correct thickness & hardness .... pads that are too hard or too thick will prevent proper contact between GPU die & cooler
Have you confirmed fans are spinning on cooler?
Set a custom fan curve for better cooling.
case fans, you need to get the hot air out of the case before the GPU recycles it?