Problem stsetch SNF 2010-10-07 13:51:44: dry etching quality difference (grass formation)

I normally use smooshal recipe for 12 min, and the etching quality became significantly worse thses days. For example, I had very good results in terms of no grass by eyes (08/25/10), but at this time (10/04/10), with the same recipe and etching time and the same oxide mask sample with same design, I could see big white circles covering majority of my wafer.
I don't know why this happens. The helium flow rate was stable to 2.47. Apprarently, the only difference to me was the helium flow rate shift to 2.47 from ~1.42.