What do you think is wrong with my car?

It's strange, upon igniting the car and taking off it's fine. It's usually fine, most of the time. However, every once in a while I'll come to a stop sign, go to take off, and go nowhere. The engine will rev for a little bit, I'll put on the gas a little more, then it will shake a lil then go up to speed and be as if nothing happened. I've been warned my transmission is slipping, but the thing is it only happens between 0-20 mph and only does it maybe once out of like 15 times. It's a 92 88 Oldsmobile Royale. With only 105 grand on it. If that helps. Thanks.