Originally I bought a D5100 because I misread the comparison chart on the Nikon website and thought it could fire a flash remotely (well, it can w/ an SB900, but I bought a SB700). So I bought a used D-90 for $600 (just a bit more than an SB900 would have cost), and I like it a lot. But sometimes it freezes up and I get F--- in the Control Panel where the f-stop should appear. Strangely, I can get the camera to function again by fiddling with controls: changing focus from AF to M; changing OS from on to off; changing the mode. My concern is that in the middle of a shoot some day this strategy will fail and I'll have a dead camera in my hands. Thoughts on this anyone?
The most common cause to the problem you discribe is oxidized or dirty electrical contacts on the lens and or camera body. Most of the time using a clean lint free cloth to wipe off the electrical contacts is enough to solve the problem. You can also use DeOxit (sold at Radio Shack) to clean the oxidation. Also make sure that all of the electrical contacts on the lens project out the same distance. They are spring loaded and can get jammed in. Good Luck and Enjoy your Nikons!