I have a 1991 300 TE–24V M104 engine with the following symptoms:

• Use the vehicle up to warm (doesn’t have to reach 80 degrees C), sometimes only a 15 min drive
• Leave the vehicle for 30 mins – 1 hr (this is vital since if I restart the car immediately, there is no issue)
• Upon restart, sometimes the car would misfire, but not all the time
• Sometimes switching the car off then starting it back up immediately would make it normal
• Other times it may have difficulty starting, or misfire would continue and car would have no power with low idle rpm and rough running. (Some times I would restart 20+ times and misfire still continues, I would have to wait a longer time to let it sit till it goes normal again)

Parts that have been changed since I owned the car:

• 8 spark plugs (correct Bosch non-resistance)
• One of the two fuel pumps (since the other is normal)
• Fuel accumulator
• Idle control valve (Bosch)
• Air flow Potentiometer
• Distributor cap and rotor (both Bosch)
• Bosch coil
• Camshaft Position Sensor
• ECU switched to another used unit
• OVP Relay switched to another used unit
• MAS Relay switched to another used unit
• Crankshaft position sensor cable switched to another used unit
• HT Leads seem to be new (Beru)

Can anyone help guide me as to where else to look for? I’ve tried going to several mechanics without any results since they have difficulty in replicating the misfire issue, since sometimes just by restarting the car immediately it becomes normal.

The symptoms you're experiencing with your 1991 300 TE–24V M104 engine could indicate several potential issues. Here are some possibilities to consider:

Fuel Delivery Problems:

Check the fuel system for issues such as a failing fuel pump, clogged fuel filter, or fuel pressure regulator problems. Inconsistent fuel delivery could cause misfires and rough running.
Ignition System Issues:

Faulty ignition components like spark plugs, ignition coils, or the ignition control module could cause intermittent misfires. Inspect these components for wear or malfunctions.
Engine Sensors or Control Module Problems:

Malfunctioning engine sensors (like the crankshaft position sensor or camshaft position sensor) or issues with the engine control module (ECM) could cause irregular fuel and spark timing, leading to misfires.
Vacuum Leaks:

Vacuum leaks in the intake system can cause rough idling and poor performance after the engine heats up. Inspect hoses, gaskets, and the intake manifold for leaks.
Coolant Temperature Sensor:

A faulty coolant temperature sensor might provide incorrect temperature readings to the engine control module, leading to incorrect fuel mixture and performance issues when the engine is warm.
Throttle or Idle Control Problems:

Issues with the throttle position sensor or the idle air control valve could result in low idle RPM and rough running when restarting the engine.
Given the intermittent nature of the problem, diagnosing the issue might require a systematic approach. Consider performing diagnostic tests such as checking fault codes using an OBD-II scanner (if available for your vehicle), conducting a fuel pressure test, inspecting ignition components, and verifying sensor functionality.

Whether generated by NS or AS, we are going to see a proliferation of one BS posting being answered by another BS posting. Ain't computers wonderful!!