Woa, hold on a second here.
If you are not overclocking your card, and have not ever overclocked your card, and you are having problems with your expensive video card, you do NOT want to go and void your video card's warranty, which is what will happen the second that you remove the stock cooler on the card.
If this problem persists, get in touch with ATi about it. Contact them, and explain your problem - your warranty covers a video card that does not work properly.
For all we know, there is something seriously wrong with the card, a flaw covered by your warranty. If you void that warranty, and the problem persists, you're out of luck, and stuck with a very expensive bad part.
If you want to try improving the card's cooling, leave the card itself alone. Instead, start by improving your case airflow. An 80mm intake fan, and an 80mm exhaust fan will improve airflow signifigantly, and may alleviate your problem if it is heat related.
I very strongly reccomend that you do NOT alter your video card in any way (including overclocking), untill you resolve this problem.
It is very important to always make sure that all of your hardware is fully functional, before modifying it or overclocking it. The first three days, running a new system build, or part, at 100% stock settings, has saved me a lot of misery and money in the past.
Although many overclockers view warranty services with contempt, warranties exist for a good reason - sometimes you simply get unlucky, and end up with a bad part.