I have one last suggestion and this goes for any equipment--if your problem doesn't go away with some of the suggestions here, have your camera checked at a certified repair facility. I actually regularly take my Nikon in for sensor cleaning and a general inspection to make sure nothing is broken, dirty, or damaged. Sometimes they find something amiss. If you take a lot of pictures it is not unreasonable to get your camera checked out before a big photography trip or at least once every six months. I find that the cleaning and occasional repair keep my camera in top condition--I use it to take thousands of images each year, and occasionally it is subjected to some rough handling. (My Nikon D80 fell off the back of a horse on a hunting trip--ouch!)
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