it could have a rev limiter while in park. My 2001 lumina has one at about 4000rpm's. I also suspect a valve isnt seating correctly, or is bent. Did you check the timing lately? Could the timing have retarded. I hear that if a belt retards by one cog, that the motor will run, but will run very very bad. But I suspect that the valve isnt seating correctly. Change your oil.