[REY 427]

I'm experiencing what I believe is my clutch slipping. I was cruising down the interstate and went to pass slower traffic. I didn't down shift and kept it in 5th gear. I floored it maybe 75% down and the RPMs immediately jumped up as if I was spinning the tires. I started to slowly let off and the speed finally caught up to the RPMs. I immediately knew something was up. I tried it again a short while later but this time in 4th gear and it did the same thing. Am I correct to assume that the clutch is slipping? I don't experience the other effects of the clutch slipping on lower gears. It's not rough coming out of 1st gear, the fluid is full, no burning smell. Is it common for this to fail with with such low mileage? The car has 4,000 miles. I'm running a Roush 427R with a T56 6 speed. Thanks for your help!