1998 Dodge durango wont shift?

i have a 98 Durango with a 318. When it gets cold (happens more in the cold than in the warm)its takes a while for the transmission to shift from 2nd to 3rd (its an Auto trans). its almost like it has to warm up before it shifts. i can go at least a mile and it still wont shift, sometimes i have to give her alot of gas and alot of rpms to get it to shift. Any Suggestions? Trans fluid looks fine and no i will not add lucas transmission goop.