I start the car at any given time, drive for around 3 minutes and the transmission slips. I shut the car off and on and the transmissions starts working again for about another 3 minutes. This occurs when the car is either hot or cold. When the transmissions slips, I accelerate the car and the trans does not make any sound. Please advise. I've checked the trans oil, it looks dark and does not present any odors other than trans oil.