2002 Buick regal 3.8L.
Question = engine won't run over 4000 rpm with out missing. Runs good on flats and hills as long as you keep the rpms down. Starts good.
There are several things that can cause this.
1. Low fuel pressure
2. Faulty ignition components such as coils, wires, plugs.
3. Faulty MAF sensor. Usually will have a check engine light and codes though.
4. Plugged catalytic converter.
If the above things have been checked, then what you describe sounds like a plugged catalytic converter. If that is the case, GM has a special policy that may apply to your car that will replace the converter at no cost to you. This depends on the VIN # of your car. You are within the mileage range for this special policy, if your car is within the correct VIN # range.
2002 Buick Regal Misfire On Hard Acceleration