mining with radeon hd 6670, 7850 and 7850 oc

7850 starts at 700kh/s then slowly decrease and after some hours is at 150-160kh/s

I've 4 machines mining scrypt on GPU. Im every machine BFGminer at startup has great performance with really high hashrate. Hashrate slightly decrease passing hours till it stabilize at a really lower hashrate compared to startup.
Could it be normal behaviour?

Thanks