My initial thought was that the PCM is going offline based on the theft lamp illumination. Ford PCMs can go offline for a few reasons.
- Vpwr interruption to the module. (This is the ignition switched feed to module)
- B+ interruption to module.
- Mainsupply voltage being too low or too high.
- Vref shorted to ground (Vref is the regulated 5.0v reference used to power the module cpu and engine compartment sensors. This includes the DPFE, IAT, CHT/ECT, TP, Fuel tank pressure sensor. Intermittent shorts to the Vref circuit will cause the PCM to go offline until the short is eliminated. (DPFEs are very common for this… I am NOT TELLING you to replace this. Someone needs to do some testing.)
- Module failure (Do not replace this without proper diag)
All modules will NEED scanned for DTCs before any work is performed. (Just because the engine light is not on, does not mean that the PCM has not identified and stored a DTC)
If no codes are present, I would typically then connect a scan tool to the vehicle and would then evaluate the PCM live data display. While in live data display, information like RPM, TP v, IAT v, CHT/ECT v, sync, Vpwr,Vbatt, Key state, Start enabled/disabled and evaluate this parameters during the event. If the live data is lost during the event, that is a really good sign that the module has gone offline. (This is how I would do it when equipped with diagnostic equipment… Not sure what you have available or how hands on you are.)
At that point I would connect a DVOM or o-scope to the VREF circuit and would watch for voltage high/low concerns. (Vref should be 5.0V) if dropping low during the stall/stumble, I would begin to isolate the components using the vref one at a time checking to see if the condition is eliminated. V-ref shorts can happen in the sensors, issues can happen within the wiring harness.
If vref is going low, you cannot forget about base powers and grounds to the pcm. I can write a book here on how to chase this down, it all depends on how hands on you are and what tools you have.