OK I have a 2000 xterra with an auto trans. It quit going into gear all the sudden one day last week. If I shut it off and start it back up it will go right into gear. If I drive it for a block or so it slips its self into neutral. No gears work. I checked the trans fluid and it was brown. Two weeks prior I put new radiator on it and had to top off the trans fluid and it looked great. What could this be. No check engine light or flashing overdrive light.