What you are seeing is a common problem which I encountered on 3 separate displays.
I have since diagnosed the problem and am currently life-testing a working solution. The real problem is very poor design of the negative voltage generator circuit for the display contrast. Everything else is working fine except that circuit. I have designed and installed in my treadmill a replacement circuit that is much stronger and am currently testing it.
Email me at zubatcha(at)gmail(dot)com for more information
