Pump Mounted Driver - Fuel Solenoid Driver Module
The PMD (or FSD as it is also sometimes known) was the single-most problematic component on the entire 6.5TD powered vehicle. For a long-long time I recommended every owner install a remote mount kit for the PMD and have a spare "known-good" PMD as a spare... Think of it as no less important than a spare tire. A remote mount kit allows you to swap the PMD on the side of the road with nothing more than one Torx bit tool and a flashlight. A spare known-good PMD also provides you with a valuable troubleshooting aid in times just like what your truck is experiencing now.
Sure, an ignition switch can be a cause of stalling, but I've only ever heard from a tiny number of owners who identified that as a problem... in 25 years and more than 100K emails/phone calls. A simple volt check of the associated IGN/ECM fuses when the truck stalls would be one way to verify that possibility. A wiring problem (pinched or damaged during some other recent service) or a corrosion problem that affects an electrical connector (usually associated with road de-icer) has also produced a few identified causes of intermittent engine running problems... a few.
I recommend only the latest gray colored Stanadyne module. If you buy a new module, be sure to obtain a new calibration resistor to go along with it. I recommend a #5 resistor. If your DS4 injection pump had/has a black module, you'll also need a new pigtail harness for the pump... the connector design is just slightly different. You'll need to remove the intake manifold to replace that pigtail. The Troubleshooting & Repair Guide shows how to properly and safely R&R the intake.
Jim