Can you tell me what is going on with my 2008 Shelby GT 500

To whom it may concern:
I have a 2008 Shelby GT 500, I noticed yesterday as I passed another vehicle on the interstagte that when I was in sixth gear, the engine revved really highly like the transmission was spinning and the car hesitated somewhat then it started to move. This conncerned me, because the car never did this before. As I was driving, I would tap the accelerator pedal, and I would see the tachometer's needle go to the higher rpm's but the car would move right away, and the "kicker" was when I looked at the boost gauge needle, it didn't move, which it should move once you get into gears 4-6. When I would see the tachometer's needle move to the higher rpm, I would notice the shift light come on, but I was already in the final gear. Gear 6. I took it for a spin today, and when I put the car on the interstate and really punched it, I heared the engine screaming, the boost gauge moved very little, and the car didn't respond right away to the pushing of the accelerator right away. But once it "caught" it then moved. I don't excessively drive my shelby, I never dragged race, or have done anything stupid or foolish in the car outside of drive it to work and back. Could you please tell me what could be wrong.I hope it is not a clutch issue, because a clutch isn't covered by my extended warranty. Could something else be the culprit? Any help you can give me would greatly be appreciated.