Problem stsetch SNF 2010-10-21 00:28:00: SF6 MFC not always stable

For DEEP recipe as example, SF6 = 130, which is controlled well for about half the etch cycles, while on other cycles it oscillates between 120~140. This is not the initial transient, but happens during the entire 12 second etch cycle. Sometimes even a steady ISO recipe will see the SF6 variations.