03 g35 sedan transmission slipping sometimes

[35sedan95]

i have a 03 g35 sedan. sometimes its drives perfectly fine and the tiptronic works perfectly and sometimes when taking off the car slips in first gear and you need to get on it to get it moving as well as the tiptronic not working at all. When i start the car and go straight to tiptronic it also says that its in 4th gear. what can be the problem. Transmission, some sort of sensor or the torque convertor. thanks