HI, my wife and I have a D90 with the 18-105 lens. We had the camera about 4 months and it's been working great until this last weekend. I was taking several pictures on Saturday and all of a sudden it just stopped taking pictures for a minute and then would work again. It kept doing that off and on that day. Then the last few days I was looking at it and the same thing happens, only thing that comes up on the screen on top is a F -- and it just blinks? I'm not sure what happened any ideas? Thank you.