Anyone can explain my VGA problem?

[puppy1014]

I got 6970 vga and run by pocblm. first time i ran it with default and got 365Mhash/s but when i overclocked it by turn down mem @ 900 and up core clock @ 940 Mhz it go down 320Mhash/s . I don't know why or i got some mistakes when i set up my rig?
P/s : -device=0 -v -w 128 -f 60 my setting in bat file.

[swusc2]

How far did you down clock the memory speed?