1995 isuzu trooper transmission problem

i have 1995 trooper auto 4x4 ls. When i drive it the transsmission is just find for anywhere from 1 mile to 150 miles. then all of the sudden the transmission light comes on and it drives just fine until i come to a complete stop. then when i begin to move it act like its underload if i leave in overdirve until it gets out of first gear then its back to normal shifting and downshifting till i come to a complete stop(the light is still flashing) or i can manually shift it from first and it is just fine. if i turn the car off and restart it the light will not be on and then at sometime the light will then come on. please help as i reallylike this vehicle