Just keep trying to set lower and lower intensities. Phoenix miner doesn't seem to be affected by negative aggression (though looking at the kernel I don't see why it wouldn't..), but at AGGRESSION=0 and a 3.6 GHz OC'd Intel Q9550, I'm seeing about 70% GPU usage, but also there is a huge hit to performance, seeing only 100 mhash on 900MHz core 5830 instead of about 290 mhash at AGGRESSION=10 and about 260 at AGGRESSION=4
If true that is horrible way to go.
If you are seeing 70% GPU load and only getting 100/290 = 0.34% effective output that is very inefficient.