Is the transmission actually slipping? Are the shifts normal or do they flair (RPMs jump up during the shift)?
I would check the fluid level and condition and if it is OK. I would then look at my VSS and ignition system sensors and wiring. The PCM compares the engine RPM to vehicle speed for each gear selected and if they don�t agree with the calculated values it sets a slip code. Bad inputs can trick the PCM into thinking a good tranny is slipping so check it out before condemning the tranny.
jeff
