weird shifting\power loss question

I just experienced a weird problem today with the way my car is shifting. When i come to a stop or put the car into drive and push the gas pedal to go, the car will start to go and then all of a sudden the engine revs high and the car feels like it has no power, the car goes practically nowhere. Then it kicks into gear and feels like there is power again then it will happen again. Its on/off, on/off. Sometimes when i come to a stop or start to go again i here a loud thump noise and feel like im going to eat my steering wheel. Can someone please tell me whats going on. Thanks