KM 2050 dark in multi copy

I have a km 2050 that prints fine for the first 40 copies or so, then is dark background. if it sets for a short time the next run will be fine for the first 10-20 copies. The more it is ran the sooner it gets dark background. Everything is clean, Corona, cavity, Xfer roller. Drum is 2 months old.
Thanks
Gar