[ant7]

The exhaust back box depending on the type fitted can be very restrictive especialy when you get past 5k RPM, Mine is the 3.2, but I was very surprised at how much faster it would rev and hit the rev limiter when I removed the standard back box and fitted the Bisimoto rear exhaust, not for everyone I know but for the time I had it fitted to the car it was great fun!
As others have said; make sure you are getting full throttle before you suspect anything else, it is also important to have a good rotar arm, distributor cap, and leads for high rev's as they can deteriorate with age.
Hope this helps.