Cells spontaneously detaching? - (Dec/03/2012 )

HI!
I am using mixed cortical cell culture plated on poly-L-lysine. In past 2 weeks cells would spontaneously detach overnight and looked like a ribbon floating in the flask in the morning. There is no sign of infection, and previously they looked very good in a flask. Have no idea what is going on, especially because the protocol I'm using worked perfectly for 18 months.
I would appreciate any advice.
Thanks!

check for mycoplasma contamination