4x4. Isuzu trooper

It sounds like you have some wear. I don't know much about automatics but I found a youtube video with someone adjusting the bands on a 4L30E transmission which is what our troopers have. If it slips in 1st and second but then drives fine in winter mode then it's a good bet that's what you need to do. By the way all its doing when you put it in winter mode is starting out in 3rd gear from a dead stop. Before you go and try adjusting your trans you will have to lift it, support the trans from beneath, and remove the supporting crossmember and remove the pan. So be prepared to do an entire flush and fill by the time you finish.