BEAS-2B cell culture and MTT assay variability

Good evening, everyone.

Does anyone have experience with culturing BEAS-2B cells (broncho-epithelial cell lines)?
I’m trying to do a MTT assay since September with cisplatin at stock concentration 3.33mM (serial dilutions of 25, 12.5, 6.25, 3.12, 1.56 and 0.78 uM respectively) but I get a lot of variability on the triplicates!

Any suggestions please?