[David M]

11.79 volts is ok. 81C for a GPU is pretty normal. They run hotter than CPU's. Your Catalyst software should be bumping the fan speed up on its own as the GPU temperature rises. You can run your cards fan at 100% manually just to see if it is capable of dissipating enough heat during the stress test. You blew the card out with compressed air to make sure it is dust free right? You want to blow all the dust out of your case before doing any of this to eliminate that possibility.

Did you get a crash during the stress test?

The value with Furmark is that it puts a big load on your graphics card.

Open up Speedfan and click on the Charts tab. Then under Analyze select Voltages. Then select 3.3, 5 and 12. Do the stress test and then go back to the charts and see what voltages were recorded during the stress test.

Or better yet, if you have a digital multimeter, use that to measure your 12 volt rail during the stress test. The 3.3 and 5 volt rail are usually not the problem with a card that is under voltaged since the vast majority of the power to run a card comes from the 12 volt rail.