98 transmission problem

I own a 1998 grand caravan with 180000 miles on it. A 4 speed automatic.My problem is when we r driving and coming to a stop the transmission slams or jerks in to gear.Once in a while when we stop then go it will not shift out of 2 gear. The way we fix this is we stop,turn car off then restart the van and it will shift again,but stills has that one problem when it down shift. I change the filter and fluid on it and still the same.What could be the problem with this.Please help