Welcome to Nikonians and the D700 forum. Sorry to hear of your problem. It sounds like there is something wrong with the multi-selector switch on your camera. While the functions of the Multi-selector can be changed in the Custom Settinngs Menu (f2 & f3) both only affect what happens when the Multi-selector button center position is pressed.
Have you tried pressing just the bottom edge of the Multi-selector just in case you are inadvertantly pressing the switch down in the center position? Good Luck and Enjoy your Nikons!