You may need to drop your memory speed to hit 900 on the core. I have mine running at 900/1800/2000 @1.1v. I have not done much testing to see how much I can lower that voltage, but my temps are fine and my GPU is stable at this. If I go to even 915mhz, I get driver crashes.
Mine needs 1.1v for 900 core. I also had to switch my setup from a P182 to a Stacker 832 with some 140mm fans blowing over the card to get it to run stable. I'd like to try more voltage but I'm afraid if the VRMs exploding.