2900xt probelm and crashing

im going to lose it ! i installed a new aftermarket gpu heatsink the thermalright hr-03 on my hd2900xt , before the card work perfectly, but just over heated a bit much, so now with this heatsink, the card idles at 42C 10 degrees cooler than the stock, but as soon as i try to play a game or 3dmark06 it crashes the system all together, and the temperature sky rockets, higher than ive seen before. i dont get it, i reapplied the thermal paste, 92mm fan on the sink, fan above card, one below card, still same results. i dont get it, any ideas

well i went back to the stock cooler, and it works great again, that thermalright hr-03 is crap, despite its good reviews. i managed to modify the stock cooler, and im getting 43C idle at stock 25% fan cycle just by removing the back mounting plate on the card, and one of the slot divider bars in the case, greatly increased the ease of air flow.