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More detail would help, such as what are you working on, and fuller description of symptoms. If you are saying that as the engine revs, power seems to drop off, there are many possibilities. Clogged exhaust system (think bad cat converter), or bad ignition module (they have a low speed section and a high speed section), inadequate fuel delivery, gas pedal not opening throttle all the way, or if your vehicle is "fly-by wire", bad gas pedal section.
You are hitting the rev limiter top range, which cuts off your ignition.
I have a 2001 Grand Cherokee and the problem drives me crazy, but it is the way the car is designed- as soon as you hit the rev limit, it shuts down ignition and your jeep will "buck" - a real pain when you are trying to accelerate into traffic!
You have the 4.0L straight six engine, yes?
If so, please take my advice and learn to live with the minor annoyance of the rev limiter- that engine is IMHO the best Chrysler ever made (nearly "bulletproof"!) and it was so reliable you cannot even get it in a jeep now! For decades, that engine was used in welders, tractors, generators...and yes, jeeps... and it is a favorite among mechanics for its durability!
The vehicle has a rev limiter. Will it rev over 3500 when you drive it? If no, then I would guess that the exhaust might be partially restricted. But the engine computer won't allow you to free rev the engine that high in park and neutral.
The computer will not allow the car to full rev in park to protect the engine. Likewise there is a rev limiter when in drive which will shut down injectors at high engine speeds usually around 5000 RPM or so.
Check your mass airflow meter. If it's bad, at least on my 1990 300ZX, the car won't rev over 2400RPM (it's a safeguard built into the ECU of the car). Grab one from a scrapyard and swap it in and see if that fixes the issue.