Another test would be to run with the side door off. Most people with 900s run the fans at low, maybe medium and have no heat issues. It has really good airflow.
One thing the 900 is noted for is pulling dust in. Have you thoroughly dusted your CPU , GPU, northbridge and southbridge heatsinks? Letting those get dusty will substantially reduce their ability to dissipate heat.
Download "Precision" from EVGA.com and use that to monitor/fan control your GX2. It will work on all Nvidia cards, not just EVGAs.
Like Zipzoom said, the GX2 is a hot power gobbler and likely a part of the issue.
You do have that case up on the desktop and not in some kind of under-desk enclosure?