If it's slipping in third and you just replaced the clutch pressure plate assembly I would be looking at the pressure plate, sounds to me like your NOT getting the clamping force you should be from it whether it be because there is something wrong with it or you have the wrong one for the vehicle combo your running. Not all pressure plates are created equally. You will not expierence slippage from a transmission as dgc333 commented on, it will either not go into gear or pop out of gear or growl, howl , or grind. But not slip.93toyota said