i get the exact same noise profile when i underexpose - i typically see this when i fire off a load of shots and my flash batteries are running low and it doesn't recycle in time - as you've seen from a few of my shots in the people and portraits forum, i shoot a lot of photos in a woodland environment where the lighting is variable and this can be enough to confuse the camera's meter - particularly if i've also AE locked and the sun has gone in!
you can sometimes help by matters simply by dialing in a little exposure compensation. by how much depends on lots of factors, but never more than a stop in my case.
but your camera is normal.