You really answered your own question. If the fan is operating as in trying to cool your gpu, then its doing its job. You already noticed the reduction of fan speed doesn't help matters, thus I suspect the fan is again doing exactly its job. Overall, you're whole PC needs to be cooled as the "airflow" isn't getting the heat out fast enough. Either improve the case airflow setup or mount at least a slot card fan to directly exhaust near the gpu. Replacing the gpu fan won't necessarily help as it still needs to exhaust the heat. If your PC case doesn't have a "side fan" maybe you should think of mounting or making one. Your gaming is sooner or later will defeat your current cooling efforts. If once you case cooling is controlled, then the gpu fan may throttle down as it doesn't need to be high RPM all the time. If you case also has an empty fan mounting port, get the required fan. Make sure your case is as dust free as possible, clear vents or filters on frt. panel if need be.