More than likely it is a shift solenoid or the pump is not supplying the pressure needed to hold a gear. This could be due to a faulty pump, O-ring seal or other internal passage problems such as blockages.
Start by checking the fluid level. To do this. Keep the car running after it has reached operating temperature and park it, open the hood and find the A/T fluid dipstick, probably near firewall, and get a clean rag or paper towel. Get in the car, start it and put it into every gear. After running through the gears, check the level with the motor running and look for the, "HOT" and, "COOL" levels. It should fall in between them. Check it for a burnt smell or dark color.
After that the next step is getting the trouble codes pulled. Any Advance Auto or Auto Zone will pull the codes for free and make diagnosing the issue much easier.
Get those and get back to us and we will work through the problem.