Unfortunately it seems your display is faulty. When did you buy the device? If still under warranty, arrange to have it sent to service for replacement. I had the same problem a few days after I bought and had to send it back (as 'rkomar' correctly suggested):
The full story is here (message #17):
https://www.mobileread.com/forums/sho...=204638&page=2
It is not a bad device (apart from the old USB connection style and very slow processor), but the company has by now stopped updating the firmware and software as things show...