I'm afraid this could be many things. The fact that you say it happens only sometimes suggests a loose connection or a bad solder joint. Something in the display circuitry is most likely.
In time it will most likely get either better or worse. You can probably hurry this up by vibrating or bumping the unit.
You will probably have to send it in. Sorry. If it is still under warranty, do this soon.
If you bought it used, talk to your vendor.
