I have the same issue with my GRD I. It's a shame as it's such an expensive machine and so well built... I think it’s about 3 1/2 years old. Actually, the first one I bought didn’t focus so I had to send it back. Then for 3 years the only problem were instable battery contacts, so that at times it would close down despite a fully charged battery. Cleaning with a cotton bud helped.
Some three months ago the banding appeared. I someow have the feeling that I smashed the sensor by using it when the battery was extremely low and it closed down in the middle of recording an image due to lack of energy... But that’s just a presumption, I don’t have the technical knowledge to make more that a guess. Strangely some days later I was able to shoot a couple of fine pictures, but then the banding reappeared to stay.
I had someone at Harrison Cameras Sheffield (I was living in the UK until recently) look at it and they said it looks familiar to them but they only remember it as a problem on different brands from quite some years ago, and they hadn’t come across this for a while. He said despite the lack of guarantee Ricoh might (morally...?
) feel obligated to replace the sensor for free.
I was told by someone who works for Ricoh that the replacement of a sensor might cost around €150 – €200. Fortunatley in Germany were I live Ricoh is well represented.