You probably have an issue with either pump pressure or the torque converter or torque converter solenoid. Automatic transmissions are very hard to work on that is why you find tranmission specialty shops as it takes special expertise and knowledge to work on them as well as special tools. Take the car to a transmission shop and have them pull the trouble codes to see if it is electrical in nature. You might be looking at some internal tranmission work in the least. Don't go to a large chain like AAMCO as they charge to much. Do a little research and find a reputable shop near you and make sure they have a good record of performance. You can use the search engine on this site to find reputable transmission shops that are located near you.