What is the temp of the room your system is in, what is the case temp, and what is you core voltage set to?
At 2.8 GHz your CPU is burning around 100W under max load. It's hard to envision an AVC Sunflower with a c/w of about 0.35 keeping the CPU to 55C max under those conditions. Doing the math, at 100W the temp differential would be 35C which would mean the air at the inlet to the Sunflower would have to be at 20C. Since the case would be probably 5C over room temp, your room temp has to be around 15C (59F for US residents). That's a pretty cool room.
I have a Sunflower with ASII on my P4 1.6A and it hits as high as 60C in Prime95 at 2.2 GHz with the core at 1.7V. When it gets that high, I either get rounding errors or a hardware failure message from Prime95. It only goes to 55C running Prime95 at 2.14 GHz with the core at 1.6V and doesn't fail. My case temp is around 25C, so it pretty much fits what the c/w model would expect. I'm getting an Alpha PAL8942 to get my temps under control, but am wondering what your secret is.
BTW, I have good airflow in my case, with a blow hole in the side and an additional 80mm exhast fan mounted in the top of the case.