Just had my stock clutch replaced yesterday because it was gone. What you're describing is exactly how my X was acting. The clutch slips more the slower the car is moving in a specific gear. So if you're in 5th doing 40mph and you floor it, rpm's will jump and then come back down. If you do the same thing right afterwards let's say doing 50mph, rpm's will jump LESS. Did you get the car new? If so, have you beaten up on it?

 

The X's clutch is way better than my VIII's. The clutch in my VIII was smoked at 9k, got 18k on my X and it's still grabbin great.