1995pontiac,firebird6cyl,sputters at 2000rpm

Sounds like an ignition misfire. Is the check engine light on ?

Sound like it needs a tune up, replace spark plugs check high tension cables , make sure there is a good spark to the plugs, clean or replace air filter, replace fuel filter , check fuel pressure and flow. check fuel injectors for leaks.

Any tune up issue could be bad catalytic

don't spend money on a tune up until you have the fault codes read
The ECM does a tune up every 60 seconds and any faults are stored as fault codes or engine light if serious enough
you can check the fuel pressure ( 45-65psi) or run quality injector cleaner as a maintenance program

Thanks for choosing FixYa and welcome Chris. This is a sign of low compression. You need to check the stored codes in your computer. Your check engine light should be on. You need to scan your computer for codes. Is it on?

Hi,
Could be ignition coils failing or injectors. Don't know the mileage on your car, but could be worn out engine too. Change your oil often?

Most likely the vehicle needs a tune-up. When in drive there is torque applied to the engine. This causes the engine to work harder to maintain the idle RPM's. When it's placed in neutral this added strain is released.

Most of the time a complete tune-up (spark plugs, wires, air filter, distributor rotor if equipped, and fuel injector cleaner ran through a couple tanks of gas) will take care of this issue. If not it's a good idea to have the engine fault codes check just to be on the safe side.
Hope this helps.

Clean out clogged fuel injectors

Too bad you are not across the street from me.
The injectors are 'fired' by power transistors in the ECM that can be individually replaced but I doubt the dealer has that capability. It could also be a failure in the control circuit before that last stage but one can't tell without a little measurement skill.
If it is only a failed power transistor, they cost < $5 US.

Did you change the spark plug?? Your car (1993) has a distibutor assy, which means it has ignition wires for each plug. How are your wires??cap?? rotor?? Have you ever done ignition tune up?? If the answer is yes and you have already installed new injector and pressure regulator (althoug it still could be a injection issue/ECM or computer, because ECM is what drives the injectors. If the ECM or wiring to #1 injector has an issue, the injector will not fire off and put gas into the cylinder, resulting in a misfire.) Remove #1 injector connector and check to see if you have power/12V on one side of the connector. If you have 12V there, than check the other wire for continuity to the ECM. You will need wiring diagram to find the right wire color at the ECM. That other wire goes into the ECM to the driver which opens and closes the injector depending on your demands as far as gas pedal input. That driver may be bad, causing the injector not to shoot gasoline. Than I would be looking into a engine issue, such as burned valve, or head gasket problem (coolant getting into #1 cylinder, flooding your spark plug). Tough to diagnose over the computer and not looking at it my self. Let me know what you find.