We had the transmission rebuilt and replaced about 25000 miles ago, and now it does not always want to shift from 1st to 2nd. It is an automatic, 4 speed, 4X4. I replaced the filter and fluid yesterday. It was thick and goopy and it does seem somewhat better. When it does shift it is smooth and runs fine, but sometimes it just doesn't want to shift out of first. If you go above 25 MPH and it fails to shift, then it "free wheels" and acts like it is in neutral. Any ideas? I hate to think we need another new transmission. Otherwise it is a great car. Is there a sensor or some sort of mechanism that tells it when to shift that may be clogged or need replacment? When it does shift, then there is no problem going thru all the gears and maintaining normal operating speed. The problem seems to be worse after the transmission is good and warmed up.