help with stable transfection screen - (Oct/07/2007 )

Hi! I am trying to make a stable transfected cells with Cos-7 cells. After 24 hours of transfection (lipofectamine 2000) in Cos-7 cells, I splited the cells into 100mm dish with G418 (0.8mg/ml) selection medium. Then I waited for a couple of weeks until my control (non-transfected cells) was dead with G418 and my transfected ones started proliferating. I then screen the postive clones by spliting the cells into 96-wells (1 cell/well). However, I stucked in this step. Each time, after I splited cells into 96-wells, cells are always dying after a couple of days incubation with G418 (0.8mg/ml or 0.2mg/ml) even they started proliferating in the first day or two. I repeated it a couple of times and same thing happened. It seems that while the cell is in single and it just could not survive under growth pressure(I lowered my G418 to 0.2mg/ml and raised the serum to 20%, but nothing changed. Cells are pretty happy before I splited into 96-well.) Any advice for this problem? Thanks.