When starting engine (when Engine is Cold), and Shifting (Automatic Transmission) into gear either to Drive or Reverse, Xterra stumbles and cuts off....I will let engine warm up around 10 minutes and shifting is back to normal...anybody knows what exactly is the Problem..according to Code P0731..possible causes would be: 1. Low Transmission Fluid Level, 2. Dirty Transmission Fluid, 3. Shift Solenoids, 4. Transmission Mechanical Problems, 5.Transmission Hydraulic Control Circuit.