I was wrestling on this in the past couple of days. In the end, it appears I have memory leak or something.
I ended up restarting the computer and surprisingly my hash rate is back to normal.
Much appreciated for the help, BenHur and Traderman.
John
My Scrypt Sleep level is 80.46%,now in 68th.
Sometimes the video drivers got error,and the mining speed can't reach normal speed.
Uninstall the old one with amd cleanup util and reinstall the new video driver may help.