my D300 has suddenly started locking up randomly. When shooting and when turning on it defaults to F4 and can't change aperture, sometimes it will fire in F4 but not focus, not AF or even manual. Tried new battery, different lens, different CF, updated firmware to 1.11, and cleaned contact points on lens mount. Sometimes if I hit the DOF it unlocks it also if I go into live view and back, it helps but not always. Has anyone had a similar problem? Any help appreciated.