97 plymouth voyager trans quits

OK here is my problem. When shifting into gear the car will move. sometimes in 10 feet sometimes in 1000 feet. Trans goes like it is in neutral. Motor can be reved several times and it will go and not cause anymore problems. sometimes putting it in park and reving motor and putting it back in geat will give the same results. This is always when trans is cold. works fine when it is worm. it is a 97 plymouth voyager. 3.3 with 4 speed trans. changed fluid and filter. This did not change the problem. is there a relay or a sensor that controls the shifting. Anybody have this problem. Any ideas would be appreciated.