What are you considering as "high RPM"?? If IRC, these cars have a rev limiter, in neutral and pushing up through the gears. Could that be what you are recognizing? Have you been able to achieve the high RPM's previously?
The sputtering does sound like it could be a rev limiter.
Definitely ref limiter. I agree. Now stop trying to impress your friends by revving it while stationary and impress them revving it in motion! Also IDK if it makes a difference but my 2006 2.4 is limited to 4000in park or neutral!