did you try flashing the device?

I think I have flashed since then but I didn't do it for that particular problem. I have not seen the problem for a long time so maybe that helped. However I think I did see it on very rare occasions after I flashed it. Can I ask, what is the temperature like where you are? It may be temperature/voltage related. I often see this type of corruption in computers when graphics ram is not cooled or under/over rated voltage. However my post is from January which makes me believe that temperature isn't the problem (unless I was in a sauna at that time ).

That means Nokia still are making undefetable Phones!!!
Friend of friend have old Nokia phone (manufactured much before Smartphone era). He get with his phone to Death Sea (much better conductor than just water or just salted water) and his phone stopped working ... but only because the battery were hurted - after changing the battery phone worked perfectly.

Same here - another smartphone would be long dead after such event - but N900 still works - and works almost correctly.