I recall a similar incident, very rare, but it happened to a D70 owner. It seemed that replacing the component boards related to the CompactFlash read/write fixed the problem.
Edited to add: the other possibility is that the camera has been subjected to high humidity and moisture has made it's way into the sensor/chamber. There was a bad batch of Sony CCD sensors a few years ago that spurred a recall, but it was only affecting compacts. However, the symptom photos shown look very close to what you have. For that, you could try sealing the camera in a bag with silica packets for a few days and see what happens.