Transmission Fluid Leaking problems of the 2006 Mini Cooper Convertible

While driving to work on hwy 270 going south between manchester rd and dougherty ferry rd in rush hour traffic sometime between 4:30 and 5:30 pm my 06 Mini Cooper Convertible was reving up as I press the accelerator but yet barely moving. Right before the the dougherty ferry exit it "kicked" into gear and began driving at about 40-50mph. I thought I may have accidentally hit the gear shifter and put it into sport drive but when I looked the gear was still in drive. So as I was traveling to work in arnold, mo the transmission would rev up and then "kick" into gear. When I got to work I explained to a CO-worker (initial "j. R. ") what happen while I was driving and we went out on our break to take a look and see if I needed transmission fluid. We were unable to find the transmission dipstick so on my laptop I tried looking it up to see where it was located. We learned that's there's a plug underneath but the transmission is "sealed for life" meaning it shouldn't really have any leaks. So I called home to let my wife to let her know that I was going to try and make it home in the morning when I got off at 6am. After driving up to hwy 270 and manchester going north at about 630am traveling at about 40 mph on 4/4/13 the car begin revviing up again whenever I pressed the accelerator. So rather than go home I headed straight for the Mini of st. Louis dealership. I got to the dealership before they opened and didn't have an appointment so they stated they would try and squeeze me in. So I went to drive the car up to the entrance to the service bay and the car was "bucking and jerking" and barely moving. Upon talking to the service department Mini I was informed that I needed a transmission and it was $8k but if I wanted to they could get me into another Mini right away. On April 9th my wife purchased a new 2013 Ford fiesta in place of the Mini.