Just purchased car. Seems when cool, everthing is fine. After warms up, the car shakes hard when it shifts... sometimes. Kinda like a wheel hop. It is really bad if I accelerate hard. Sometimes is does it when accelerating normal. Sometimes it is fine. A light comes on dashboard when I start the car. Say "low trac", and then goes off. I do not know if that has anything to do with it. Light doesnt come on when the problem occurs. Where should I start?