Very high temps with aftermarket cooler

Hello,

I have an ATI Radeon HD 5870 that I decided to installed a third-party cooler on to hopefully reduce temperatures and most importantly, noise. The cooler, which is a Zalman VF3000A, did indeed make the computer a lot more quiet, especially with its' adjustable fan speed slider.
My problem is that the GPU gets unnaturally hot under load! While running FurMark or GPU-intensive games such as BF3, my temperatures are usually 80-90c. According to benchmarks and reviews of the VF3000A, common load temperatures are around 50-65c.
I am pretty sure the cooler is mounted correctly. In fact, I recently re-mounted it and re-applied thermal paste (although I always find it hard to determine the correct amount of thermal paste to apply).

I usually have the fans set to the lowest speed possible (about 1400rpm). Increasing the fan speed to maximum (about 3000rpm) has little effect on the temperature (5-8c).

Do you have any idea what might be causing these high temperatures? The cases' airflow isn't that great, but taking off the side door and even placing a table fan next to it only managed to drop it down to 75-80c.


Thank you for your help!