You said "slips at times when changing gears." Can you define that a little better? What are you doing when it slips? I'm just trying to understand if it's really slipping, or just a "mushy" shift...

EDIT: Here's a pretty good "how to" video IMO. It's on a Mazda pick up, but as he says, the process will be nearly identical on your car. Have plenty of rags on-hand, and that drop cloth. It can be messy a messy job. Don't be under that pan where the fluid can dump on you.
https://www.youtube.com/watch?v=yb_1wbiPz9g