'02 Jetta 1.8T Transmission Problems

My '02 1.8T Jetta has been experiencing some transmission problems lately. My RPMs will sky rocket over 50K but my MPH won't go above 20. I brought it to the dealership and after diagnosing discovered that my transmission fluid was leaking through the left inner axle joint. They replaced that part, replaced the axle flange seal, and topped off the transmission. The same transmission issue started again this morning. It corrected itself after a mile or so of driving.

Has anyone else experienced this problem and can give me some advise?

sounds like your clutch is slipping!