Engine cuts out around 3,000 RPM

This trucks had problem with the distributors but to be sure lets run the codes to see what comes up. Here is a guide:

https://www.2carpros.com/articles/nissan-obd1-diagnostic-trouble-codes-and-retrieval-procedure-1984-to-1994

If you get ignition codes here is a guide to help you remove it and install a rebuilt unit.

REMOVAL

1. Mark position of No. 1 tower of distributor cap on distributor housing.
2. Remove distributor cap.
3. Crank engine to align rotor with No. 1 mark on housing.
4. Place a mark between distributor housing and engine.
5. Remove distributor retaining bolt and the distributor. Do not crank engine when distributor is removed from engine.

INSTALLATION

1. Align rotor with No. 1 mark on housing.
2. If engine was cranked when distributor was removed from engine, crank engine until No. 1 cylinder is on compression stroke and the timing marks align on the pulley and front cover.
3. Install distributor into engine, aligning all index marks made during removal.
4. Loosely install distributor retaining bolt.
5. Adjust ignition timing and tighten distributor retaining bolt.

Please let us know what happens.

Cheers, Ken

It sounds like you've already performed a lot of troubleshooting and replacement of components on your 1987 Nissan Hardbody (D21). The fact that you've gone through so many potential problem areas makes it a bit challenging to pinpoint the exact issue, but I'll offer a few more suggestions that might help you narrow down the problem:

Exhaust System: You mentioned changing the catalytic converter, which is a good step. However, it's worth ensuring that there are no exhaust restrictions downstream of the catalytic converter. A blocked muffler or exhaust pipe could potentially cause the symptoms you're experiencing.

Ignition Timing: Double-check the ignition timing to make sure it's set correctly. Incorrect ignition timing can lead to poor engine performance, especially at higher RPMs.

Fuel Filter: Even though you mentioned good fuel pressure, a clogged fuel filter can still cause intermittent fuel delivery issues, especially at higher engine loads.

Wiring and Connectors: Inspect all wiring and connectors related to the components you've replaced. Ensure that there are no damaged wires, loose connections, or corrosion.

Mass Air Flow (MAF) Sensor: The MAF sensor measures the amount of air entering the engine and helps adjust the fuel mixture accordingly. A malfunctioning MAF sensor could lead to poor performance at higher RPMs.

Throttle Position Sensor (TPS): You mentioned replacing the TPS, but it's worth checking the installation to ensure it's properly calibrated and connected.

Vacuum Leaks: Even if you've checked for vacuum leaks, it's worth rechecking all vacuum hoses and connections. Even a small leak can cause issues, especially under high load conditions.

Idle Air Control (IAC) Valve: The IAC valve controls the engine's idle speed. If it's malfunctioning, it could cause problems when transitioning from idle to higher RPMs.

ECU Grounds: Ensure that all the grounds for the engine control system are properly connected and providing a good connection. Poor grounds can lead to various electrical issues.

Compression Test: Consider performing a compression test on all cylinders. Low compression in one or more cylinders could lead to poor engine performance under load.