Gear Slip / Transmission Slip

[rhales]

I have a 2001 X5. Yesterday I was driving along without any problems on the highway. Then the highway started to take me up hill. In order to maintain the speed limit while going uphill I had to "put the pedal to the metal" if you will. During the acceleration everything was going fine and then all of the sudden it decreased in power but my RPM's shot through the roof. It sounded like my transmission missed the gear or something. I let off the gas for the RPM's to come down and then gave it a little gas and it seemed to shift into the appropriate gear it needed to finish climbing the hill. Since then it seems to happen only when I accelerate quickly (i.e. entering the freeway or climbing a hill). My first thought was to add some transmission fluid and that would fix the problem but after reading some posts it looks like it has a lifetime transmission fluid that never needs to be changed. (hard to believe) Anyhow, it seems to work a lot better when I switch over to manual shifting. However, that is starting to slip too. I am not going to drive it anymore until I figure out what might be wrong. So any help would be greatly appreciated. I have experience fixing cars so if I can avoid taking it in to a BMW dealer to save some $$$$ and fix it myself as I have been hit hard by this economy. So any help or advice would be greatly appreciated. Thank you!