engine sputters at high rpm

1997 pontiac grand prix gtp...engine wont rev beyond 3500-4000 rpms...starts to sputter....no check engine light.car runs fine from idle to 3500 then it acts almost like it has a governor on it.looses rpms back down to 3000...stabilizes then builds back to the 3500 mark.have not replaced any parts yet.

thanks for any help
A.J.