Sudden drop in Hybridoma viability

[Haldiwala]

Hi,
I am culturing a hybridoma cell line. The problem I'm facing is that cells grow fine during the first four days of a batch culture (initial cell density 100,000 cells/mL) and reach upto 2-3 million cells/mL by day 5. After that cells start to die and there is drop in viable cell number every day until by day 8 the culture is finished. In short, there is no stationary phase of batch culture and from late exponential phase, cells start to die. Could anyone surmise what is going on here and how to prevent this sudden drop in cell viability. If you need more info, please let me know.
Thanks.