That error is almost always a hardware issue. Either it's overclocked too high or not getting enough power. I'd drop the OC down 10 MHz and let it run for a while and if it still crashes, try another 10. Also, with mining you don't need memory clock speed. Underclock that. I have my 5850's running at 860 GPU Clock and 300 Memory clock and they've been rock solid, getting me almost 350 MH/s. (If you're using poclbm, use -v -w 256 and -f 1 for the flags). Of course, depending on the card, your mileage may vary, but definitely try to get the memory clock lower. It'll run much cooler.
Make sure you're using MSI afterburner for overclocking as well.