I have a 7950 HD gpu that I run it off of, and I start out with 500-600kh/s, then it lowers down to 200, and then stops all together after 30mn of running. The card is brand new, and I haven't overclocked it. Why is this happening? I am mining with us3.wemineltc.com as my pool

Also, I am running GUIMINER scrypt, and my gpu temp usually stays around 80-85C. Please help, as time is money as I'm sure you all know