bad cell condition for transfection - (Mar/12/2007 )

goodmorning all
i am facing a problem these days in transient transfection, i seed my cells in 6 well plate for 5x105/well and next days usually i got 90% confluency before but these days i got may be 70% and too many dead cells?
first qwuestion can i start transfection in 70% confluent cells (i am using hek cells stably transfected with 2 plasmids)
second what is the cause of the dead cells ? may be may cells were 90% confluent before doing trypsinization and seeding for transfection? will this considered overgrowth to start?
thankx in advance

sometimes this can happen with high passage number 293 cells. If possible get new cells out or use low passage cells.

thankx scolix