Well slippage is not a good sign on a car of this age and mileage. You might try having the car scanned for trouble codes, and also changing the fluid and filter.
With modern automatics, it's very hard to try and diagnose them remotely. A good transmission shop should be able to perform pressure tests and scans and come up with some answers.
you might get lucky and just have a bad pressure regulating solenoid, or it could be worse, like internal damage. A scan might help.