Transmission problem

I have a 1995 chevy lumina car with 3.1 L and automatic trans. After I drive the car for 20 mins to a half hour, when I come to a complete stop the trans feels like its slipping when I take off again. Once I get moving it goes like nothing is wrong no here is the weird part, if I turn the key off and start it right back up, its fine again for about 20 mins again. The transmission was replaced 2 years ago and the cars has about 100,000 miles on it. Can any one please help me figure out this problem