I would be checking the alternator drive. Is the drive pulley on the alternator starting to freeze up?My 03 xterra super charger se/xe vibrates when I reach about 45mph then it stops when I reach about 55mph what could it be? My steering wheel is beginning to feel heavy and difficult to turn. Fluid level looks good. Finally, my rpm needle while driving on the highway started going up and down with the dashboard lights flashing. Does anyone know what could be happening?