I was having intermittent trouble with my new D90 recently. The dreaded blinking "ERR" in the LCD. My symptoms included: wouldn't focus, shutter would trip but no image recorded, ERR light. The only way to clear it was to remove the battery. The thing is that it would only happen once in a while so, at first, I didn't think too much about it. It got bad enough though that I decided I had a bad copy and was about to send it back for a replacement when... I noticed that the LCD would flickr when I wiggled the lens.
The solution? I removed the lens, wiped down the contacts and the mounting ring, and reattached it. Problem solved. I feel a little sheepish admitting how lame/lazy I am for not checking that in the first place. Lessons learned? I'm an idiot. Plus, check the lens mount if you're getting weird intermittent mechanical problems with your DSLR.