You should find max shader clock first, above 1458MHz is great :D To rule out core and mem, set them 670 and 1150Mhz or so. If, say, 1512 shader WAS stable Crysis 2hr, set core 50% of shader clock and test again (mem still "low" 1150)...lastly, raise mem till you BSOD , then back some :D
Also...